They might be "filled" in the sense that enough invites were sent out vs the total # of class spots, but I've had current classmates accepted last year from March up until June. People put down deposits at one school while waiting for decisions from other schools. In fact, during my first week or so of class we had someone leave (not sure why) and they replaced that individual last minute with someone off the waitlist. Don't come in with a negative attitude, there is plenty of movement on the waitlist and will be up until classes start, leave a good impression and you'll still have a shot even now.
If you interviewed a few months ago and are still on the waitlist, now would be a good time to send in update letters to show your interest, demonstrate that your GPA has been solid (hopefully) or that you've committed yourself to other tasks whether it be research, volunteering, working etc.