Some schools may still be pulling people off the waitlist during the week before classes start (sometimes during the first week of classes even), so by the time they reject everyone left on the waitlist, you probably will have already matriculated somewhere else. If you don't have an acceptance and don't get in off the waitlist, you probably can expect a final "unable to offer you a seat" letter sometime in August, but I'm not sure about that.
I'm right there with you all (kuhlguy, we've got FOUR waitlists in common). Just keep sending update letters and wait for May 15th. That's when the action should start (IhopeIhopeIhopeIhope). Hang in there everyone