Wow. I can't believe there is so much confusion here. I thought the AAMC rules were pretty clear.
You CAN be on as many waitlists as you want for as long as you want. You are considered automatically removed from all waitlists as soon as you start classes at a school. You are under no obligation to withdraw from waitlists if you really are interested in going to the schools that waitlisted you.
After May 15, you can hold an offer of admission at only one school. "Holding" and "accepting" an offer of admission are the same thing.
So, say you've been accepted at School A, and waitlisted at School B and School C. If School B contacts you to say that they're giving you an offer of admission, then you must decide ASAP if you want to attend School A or School B. The turnaround should be no more than 1-2 days. If you decide to go to School B, you must contact School A and say "I changed my mind and I'm going somewhere else". If you decide to stick with School A, you must tell School B "no thanks". Either way you can stay on the waitlist at School C.
If you then later get off the waitlist at School C, you have to make the same evaluation process.
I agree with the earlier post that it's best to have your mind made up about your rankings for your waitlisted schools so that you can give them a decision immediately.
Hope this helps