Waitlists are done in entirely different ways from school to school, program to program. I know for at least one program (a very small one, with typically 3-4 acceptances each year), they simply waitlist everyone with acceptable applications, and reject only those whom they definitely do not want attending the school. Other programs, and I think this is true particularly of "top" programs, play the numbers game... but also tend to send out later rejections, in my experience.
Because of this, I think the statistics could vary greatly, depending on how the program deals with their waitlist.
Also, as far as I know, it's acceptable to ask where you are on the waitlist; I've known several people who have asked, although they haven't always gotten a straight answer. I had a friend who found out that he was the first person on his top choice school's waitlist- he ended up getting an offer, but he had to wait until the very last second.