There really are no "deadlines"....as a waitlisted student I understand what you are going through. The reality is most people (with exception to those accepted at more than 3 schools) will put down money to save their seat at 2-3 schools and put off their decision until they absolutely need to make it (ie May, June, July, even August). The deposits at some of these schools arent a big enough deterrent to force some of the accepted students to make a choice. Seriously, we all have worked hard to get where we are and spent a good amount of money, whats another x amount of dollars to save a seat in a professional school? I realize its adding more fuel to the fire (ie spending more money) but what is worth more to an applicant: peace of mind or saving a few hundred bucks? Some might argue with me but many look at waiting to make their decision about which school to go to in this manner.
Personally, I wanted to save my seat at two schools....fortunately my parents stepped in and told me I should make a choice not only to save money but to sit down and ponder and just come to a decision (i.e. who wants to make a huge decision today anyway, when tomorrow looks so much better?)
In the end, while there are deposit deadlines, most have already passed. I know at Temple it was 4/1 for the second deposit. So, while there are actual deadlines, most of the time people can buy as much time as they want to make a decision....and usually end up waiting until 1-3 months before school starts to do so. The reality is there are no actual deadlines. A person could put down a deposit at however many schools they want to, not inform the schools they choose not to attend, and show up to class the first day at whichever dental school they wanted to attend, thus forcing the uninformed schools to not know their actual enrollment until the first day of school. This is why there is a lot of waitlist activity right before school starts, the day school starts and even the first week of class.
It sounds like pre-meds have an absolute deadline (May 15??)...maybe I misinterpreted that on the Pre-Allo threads. I know for sure PhD's have deadlines and once they choose one school they cannot switch after a certain date under penalty of losing their spot at both schools. They have to sign waivers to this effect.