At UCSF I was told that "the wait list doesn't work". And that is a direct quote. It made me laugh.
I think it probably depends on the school. UCSF, for example, is cheap and great. Many of the people who get in will probably pick it above others for those reasons. Of course, all of those people are opening up spots at other med schools. I am hoping that wait lists DO work at most schools. I imagine they do, why would they have them otherwise?
BTW There are many previous posts about this subject. You might get some insight from those.