It's tough to forecast this sort of thing...it really varies depending on the schools you're talking about and the caliber of the applicant. When I was applying I found that sometimes a waitlist offer meant that a school doesn't think you're really interested in them, even though they're interested in you. This is why it's important to keep communicating with schools you really like, because many are more likely to accept you from the waitlist if they know you really really want to be there. When I was applying a few years back I applied to around 15 schools, did 12 interviews, and ended up with quite a few waitlist offers...I think in the end I was accepted to 4 schools outright, waitlisted at 6, rejected at 4, and withdrew from 1 before the interview. But because I was accepted to my 2nd choice school and waitlisted at my 1st choice, I ended up withdrawing from all those waitlists, so I'm not sure how it would have all played out. But every school seems to have its quirks.
I actually remember a few of them specifically saying that if I wanted to go to their school I should just write them a letter and I'd probably get in off the waitlist. Obviously this isn't the case for the really top top schools, but for me it was true for places like Georgetown or UConn or BU. So the message is, write them and make sure they know you're really interested! It doesn't take much effort and it surely can't hurt!
I hope that's helpful...it's tough to know without some more information about you and the schools you're referring to.
