Halcyon23

Definitely call them ASAP. It's pretty normal to do that on the 15th. You don't want to miss out something important for courtesy's sake. 

Thanks for the advice and encouragement, guys - I just had the Skype meeting, and it went very well. Regardless of whether or not I end up getting an offer, it was a very encouraging chat to have!

For the benefit of others, please decline as many acceptances as you can as soon as possible. It will help decrease the odds that people who are waiting to hear back from a school they were waitlisted at from being put in a bad position on April 15th.
(Why am I posting this despite being shut out? I know some people IRL who weren't, so I'm still witnessing this process play out)

What everyone else said, absolutely do the meeting. It's so much better than excessively formal emails. But you can turn generic questions into more thoughtful ones. I had a skype interview a while back and instead of just asking something like "what is the funding package like for a TA?" I asked about the sort of teaching opportunities available as a TA (are they only intro courses, am I just an assistant or do I teach entirely on my own, if I teach entirely on my own how much freedom do I have w/r/t to syllabus creation). Things like that. You can also just ask for clarification/more detail about the program. Asking about relations in the department (how easy it is to meet with professors, do grad students spend a lot of time together in the office, etc.) is another good thing to ask so you know what it's like in the informal settings.

What everyone else said, absolutely do the meeting. It's so much better than excessively formal emails. But you can turn generic questions into more thoughtful ones. I had a skype interview a while back and instead of just asking something like "what is the funding package like for a TA?" I asked about the sort of teaching opportunities available as a TA (are they only intro courses, am I just an assistant or do I teach entirely on my own, if I teach entirely on my own how much freedom do I have w/r/t to syllabus creation). Things like that. You can also just ask for clarification/more detail about the program. Asking about relations in the department (how easy it is to meet with professors, do grad students spend a lot of time together in the office, etc.) is another good thing to ask so you know what it's like in the informal settings.

Declined Irvine LPS, Columbia, Michigan, Harvard, Carnegie Mellon, UCLA, UConn, and Pittsburgh. 

This. If you have a specific interest, you should start getting a firm grip on it.
However, if you're interested in a very broad overview, Analytic Philosophy: An Anthology edited by Martinich and Sosa is a great collection of primary sources for all the most relevant works from the first half of the 20th century. Not exactly contemporary but I'd say knowing the most famous works from Russell, Quine, Rawls, etc would be good. The book is divided by the various subfields and the end of each has a 'Further Reading' section that can be helpful.

I've accepted the offer from Indiana University! Consequently I've turned down the offer from Brandeis and removed myself from waiting lists at UW-Madison, Michigan State, CU-Boulder, UC Davis, and the University of Virginia. I hope that helps someone here.
It's been great to have an outlet for all the frustration this process caused, and to have more people who understand what I'm going through to share these moments with. I wish you all the very best, and perhaps I'll (unknowingly) meet some of you again in the future.
Cheers!

Here's the basic structure of mine (given how late it is in the season you may need to modify it to reflect that aspect):
"Dear Professor [Name],
I'm an applicant for the philosophy phd program at [university] and I have not received information regarding my current status. At the risk of being too forward or seeming over-eager I was reaching out to you to see if I can learn about my current status. Provided that there is no information you can give out at this time, do you have an estimate on when information will be sent out? I look forward to hearing back from you. 
Thanks,
[My real name]"

Here's the basic structure of mine (given how late it is in the season you may need to modify it to reflect that aspect):
"Dear Professor [Name],
I'm an applicant for the philosophy phd program at [university] and I have not received information regarding my current status. At the risk of being too forward or seeming over-eager I was reaching out to you to see if I can learn about my current status. Provided that there is no information you can give out at this time, do you have an estimate on when information will be sent out? I look forward to hearing back from you. 
Thanks,
[My real name]"

Here's the basic structure of mine (given how late it is in the season you may need to modify it to reflect that aspect):
"Dear Professor [Name],
I'm an applicant for the philosophy phd program at [university] and I have not received information regarding my current status. At the risk of being too forward or seeming over-eager I was reaching out to you to see if I can learn about my current status. Provided that there is no information you can give out at this time, do you have an estimate on when information will be sent out? I look forward to hearing back from you. 
Thanks,
[My real name]"

From an email response I solicited from Laurie Paul at Chapel Hill (I received her reply on Mar 25):
<<I am sorry that we can’t give you any more information at this time. At this point, we have made offers and constructed a waitlist, but we have not sent out rejections because we have not made final determinations about the applicant pool. As soon as our entering class is filled we will process rejections. I am sorry I can’t tell you anything more about your personal situation. I know this is a stressful period and UNC will be in touch as soon as it is possible to give you more information.
However, if it helps to have more explanation, I am reproducing a comment here that I made in an online forum about admissions last year:
"I am UNC’s Director of Admissions. I am aware that applicants would like more information, and I understand that this is a stressful time. However, I believe that, in UNC’s case, sending out rejection notices before the start of April is not in the best interest of the applicant or the Department. This is why we have not sent out rejection notices yet.
Let me explain: UNC does not have a “secret waitlist”, or anything resembling such. Rather, we have found that, on occasion, as our list of who is considering our offer of admission evolves after the campus visit (as prospective students with offers begin to make decisions), we need to revisit our waitlist. (We retain extensive notes on candidates at every round of the process.) In particular, we may revisit our waitlist because we are looking to have an entering class that is intellectually diverse, and as the season progresses and people start to make decisions, that diversity can be imperiled. 
The reason we have not sent out mass rejections is that occasionally, when candidates who failed to make it past an early round are reassessed when we revisit the waitlist, a candidate is subsequently brought forward and officially waitlisted or admitted. If we rejected applicants early in the way that some people apparently think we should, we could not do this. (Rejecting candidates entails that we officially process them for rejection through the Graduate School.)
Candidates who have not been offered admission at this time from UNC can infer that their chances of admission are not high. Yet, they are not non-existent, and as soon as we can give candidates further information, we will. (We expect to begin sending rejections at the start of April. However, we are waiting on our current prospectives to make decisions, and so there is only so much we can do. We are doing the best we can, and we urge all prospectives to make decisions in a timely manner.) 
My assumption has been that candidates prefer a small chance of admission to a zero chance of admission. (If they don’t, they have the power to change the situation. They can withdraw themselves from consideration.) It would be much easier to rush through the process and to send mass rejections early. But this is not in the best interests of the applicants or the Department. It is not appropriate for me to give further information about the internal workings of the admissions process, but I hope this explanation helps our applicants to understand why we have not been prompt in sending rejection notices. I am sorry that the process is drawn out and difficult, and for the anxiety this causes.”
I wish you the best in this admissions season.>>
Still not rejected or officially waitlisted, so I guess I'm waiting until the bitter end, since UNC is one of my dream programs. That said, definitely not holding my breath for a UNC offer.

Dear All,
Yesterday I received a response from my solicitation to Prof Sensen at Tulane. I have not been accepted there, which means that I have been shut out this season. I reckon I should have cast my net a bit wider, as I only applied to six programmes, but I am at a point at which I have accepted the fact that I am meant to be doing philosophy in other ways. For now, at least. 
I have really enjoyed your comradeship and company over the last several months. It would have been far difficult to survive this trying time without y'all. Best of luck to all beginning their PhD journeys this autumn, and bon courage to those whose lives will, like mine, take them on other paths for the time being. It's been a pleasure serving with all of you. Much love!
Misha

While it is a bit optimistic it doesn't seem too optimistic. I'm guessing that not every person admitted will accept the offer and that a handful of the people higher than 9 on the waitlist will accept an offer elsewhere. With that in mind, you will probably move up at least a few spaces on the list prior to April 15. So maybe it's not that they go 9 people deep, but that by the time they do pull from the waitlist, you'll have moved up to a higher position?
But this is all speculation.

I've been absent from the site due to a combination of being busy and intentionally avoiding it (I've come to believe this is an unhealthy thing for me to do, at least too frequently). I figured for accuracy's sake and for other people I should share an update on my admissions decisions.
I've been accepted to VA Tech and SF State, the former with a TAship and the latter with a teaching associateship. I expect I will decline both this week but I'm not 100% sure.
I've been waitlisted at CU Boulder, University of Virginia, UConn, Michigan State, and University of Houston.
I was also chosen by UW-Milwaukee to receive the AOP Fellowship; before I had merely been nominated by the department and it wasn't a sure thing.
I plan to decline/take myself off of waitlists for all but a few of these by the end of the month. I apologize to people who are desperately waiting to hear back from them.

Finally heard back from UConn this morning. I'm on the waitlist but if accepted I'll receive full funding as a graduate assistant. If you plan on declining an offer from UConn, please do so ASAP .
For those of you still waiting to hear back from UConn, nothing in the email mentioned whether or not they're done reviewing all applications. They just mentioned that they have finished reviewing mine and decided to put me on the waitlist.

Finally heard back from UConn this morning. I'm on the waitlist but if accepted I'll receive full funding as a graduate assistant. If you plan on declining an offer from UConn, please do so ASAP .
For those of you still waiting to hear back from UConn, nothing in the email mentioned whether or not they're done reviewing all applications. They just mentioned that they have finished reviewing mine and decided to put me on the waitlist.

Thanks for the update and congratulations! 

Finally heard back from UConn this morning. I'm on the waitlist but if accepted I'll receive full funding as a graduate assistant. If you plan on declining an offer from UConn, please do so ASAP .
For those of you still waiting to hear back from UConn, nothing in the email mentioned whether or not they're done reviewing all applications. They just mentioned that they have finished reviewing mine and decided to put me on the waitlist.

Finally heard back from UConn this morning. I'm on the waitlist but if accepted I'll receive full funding as a graduate assistant. If you plan on declining an offer from UConn, please do so ASAP .
For those of you still waiting to hear back from UConn, nothing in the email mentioned whether or not they're done reviewing all applications. They just mentioned that they have finished reviewing mine and decided to put me on the waitlist.

Finally heard back from UConn this morning. I'm on the waitlist but if accepted I'll receive full funding as a graduate assistant. If you plan on declining an offer from UConn, please do so ASAP .
For those of you still waiting to hear back from UConn, nothing in the email mentioned whether or not they're done reviewing all applications. They just mentioned that they have finished reviewing mine and decided to put me on the waitlist.

RIP my chances at Austin. Ah well. I got waitlisted at Davis and I'm hopeful of that, plus I still need to hear back from some other schools.

UT-Austin sent out some acceptances a while back but I don't think they've done rejections or waitlists yet. As seems to be the norm with me, I've emailed them but received no response.

I’m always just pretty direct. Find out who the DGS is and send them a short email. In my experience, most people will get back with you pretty quickly.