notJustin

It means it is likely  not an issue at this point, but be prepared to discuss it since a particularly low gre or gpa may get discussed in an interview.  Basically i would say this, some programs or people may care and they may inquire further about Why if it is an anomaly based on your other application stats why you scored so low or performed worse.  my advice is this:  be ready to cite personal evidence of why you don't think that adequately reflects you, bring up anything you've done that proves contrary.  don't give an excuse such as "I was the first person in my family to go to college, I struggled with adjusting to college, I am not a good test taker (unless you have a diagnosed issue like dyslexia), something happened while i was in college that made it difficult to succeed".I would advise not to bring it up unless it is brought up.  Handle it with maturity and remain confident.  i think a way to maturely go about discussing it would be to not make an excuse, explain what you can, show how this is different and keep it matter of fact.  

Congrats for getting to the interview stage!
Don't talk to the students about your low GPA or GRE or else when the faculty members ask the students what they thought, if they had no interaction besides talking to you about low GPA GRE, then thats all the info they will be able to pass forward.
 
Play up your strengths and just be genuinely enthusiastic about research. If you ever get in a position where you don't really understand someones field, ask them about their project, the limitations, what techniques they most enjoyed learning, etc.
 
Have fun AND DO NOT GET DRUNK! I cannot tell you how awful and how many interviewees come through and get wasted, sleep with each other, with a student in the program.. ugh.... gross. I am not joking. 
 
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How much does your GPA or GRE scores matter after a school has invited you to interview for their program?  Is everyone more or less on an even playing field, or is there still a preference towards excepting people with better stats?

How much does your GPA or GRE scores matter after a school has invited you to interview for their program?  Is everyone more or less on an even playing field, or is there still a preference towards excepting people with better stats?

If you make it to the interview stage that means you are qualified. I wouldn't fret about GPA or GRE at this point. The key is to be enthusiastic and to get along with the people currently in the program (faculty and students). And to have fun, that is important.

The subject test is only offered April, September, and October. So You've already missed the opportunity to take it.

To be honest, I doubt a 90th percentile is enough to offset a low GPA because GPAs speak for consistency and ability to handle coursework like that during a PhD. 
However it may, if you have a 90th percentile in the subject test plus a high GPA within your major (presumably related to the field you plan to enter). I think then the admissions committee will more likely look at your transcript to see if the classes bringing your GPA down are particularly relevant to the field you plan to enter anyway.

I can't find where I saw it, but I read something that had averages in the mid 70s.
 
I would expect them to be lower than the GRE which is around the low 80s. They're pretty tough and all of the advice about taking them is "don't take them." Nobody requires them and the people that do take them are applying to places that recommend taking them, which are generally only the top programs. I think anything above the 50th percentile is worth celebrating regardless.

It depends on what you're calling top-notch.  
On the positive side, who cares that your successes are recent?  You got a first author publication as an undergrad (and maybe a second on the way).  Pat yourself on the back.  Plus, your scores are solid.  
What hinders you with tip-top programs is the GPA, BUT talk to your research advisor(s).  Maybe they can recommend specific POIs you should contact at the schools you applied to.  That might make all the difference for you.  With that said, I don't think a place like Harvard would come through.  But, a solid, top 20-30 definitely might.  See if you can squeeze in a poster presentation (maybe at a regional/local chapter of your professional organization) and update your schools.  But, honestly, that would be gravy.  As things stand now, I do think you have a shot.  Good luck.