Yeah, you're supposed to give up other acceptances by May 15, although you can still be on as many waitlists as you want. The dir of admiss from Emory told me schools that start early can force you to decide by May 1. However, there are people every year that still hold onto acceptances past May 15, whether out of ignorance or spite (or something). She told me that Emory is allowed to rescind acceptances but they normally don't. I guess most people pick one when they are confronted.
I'm on a couple of waitlists with no acceptances, so I highly encourage everyone to pick one school now!
-X