You are confused by the "official numbers." BU accepted no one off the wait list in 2007 because they "over-accepted in Jan/March" They ended up with more students than they wanted. I think the same thing happened in the 2008 cycle.
It's a pretty hot school right now, for a variety of reasons. One of them is that it is a totally awesome med school in just about every way!!!
For example, their admissions process is nothing sort of genius. The student body they pull together is truly amazing: smart, dedicated, focused, great people. It is, overall, the least toolish group of people I have ever been associated with.
And Boston is great too.
And I don't think they outright accept 500, it is more like 350. But I might be wrong.
But if you are deferred, stay very hopeful. Many of us here were first deferred and then accepted in March!!