First off, May 15th is an AMCAS guideline, not a hard and fast rule. However, many schools do use that date as a cut-off to harrass applicants who are holding acceptances at their school in addition to others.
Financial aid -- many students do continue to hold multiple acceptances past May 15th without penalty because they are waiting on financial aid offers. If this is the case, it is only polite to inform all the schools where you are holding acceptances of this situation. Otherwise, you may become the recipient of letters that ask you to choose by the end of the month or else risk losing your seat in that class. Generally schools are understanding if you are waiting on multiple financial aid offers, and will extend their "deadlines" but not indefinitely -- you may end up getting an extra 2-4 weeks. Thus, IT IS YOUR RESPONSIBILITY TO BE ON TOP OF YOUR FINANCIAL AID APPLICATIONS!! Turn in your FAFSA as soon as possible for all the schools you are still considering, call financial aid offices to find out when they are mailing applications to you, call to check on your status, etc. Most financial aid offices work very quickly -- there really shouldn't be more than a 2-week turn-around time if all your info. is complete upon submission.
Finally, the only wild card in all this is admissions off of a waitlist. According to the AMCAS guidelines, after May 15th, schools no longer have to give students up to 2 weeks to decide on an offer of admission. Thus, it is not unusual for an applicant to only have a few days to decide on an offer of admission and then withdraw everywhere else, especially as it gets closer to the start of classes. I'm not sure how waiting on financial aid info. would play into this -- it probably varies on a school by school basis with some giving extra time and others choosing not to.