I posted the waitlist for Purdue (poorly, since I didn't notice that it erased my undergrad gpa and gre scores when reloading a new captcha, and it didn't like the '+' I put between admission and funding). Here's what I know: usually, they notify people who are getting a fellowship offer early (so I expect that to be the case for the first post). Also, in the past, they've notified people by email, at least initially, where rejections were sometimes also by mail and didn't usually come until after offers and waitlist notifications are out. Finally, someone on here posted that they were told they could expect to hear this week or early next week.
I suspect that they changed their admissions process for this year. Last year, they offered acceptances without guaranteed funding (waitlist for funding). I think this year they are only offering acceptances with funding, and waitlisting (for both admission and funding) the rest. In addition, though this is only on the basis on the two other posts, they may be sending out everything at once and by mail (this is consistent with the timeline offered). Unfortunately, however, there just isn't that much information at this point. I shared your confusion about when I might hear back, so hopefully this is helpful.
Edit: looks like they are notifying those on the wait list through email. Still, they may not be finished until the end of this week, or even early next week.