Seth Abramson's blog. Seth did some research into this most mysterious of topics, last year. Take a look and see what you think.
In contemplating acceptance rates, keep in mind that statistics alone may not give you the entire picture. There is a school of thinking that holds that certain programs "curate" their incoming classes so they end up with a variety of writers to complement one another and work well with the faculty. That is, your chances of admission may be better at some schools than at others, not because of a school's selectivity, but depending on whether you fit a certain mold of writer the school may be looking for.
Edward Delaney in The Atlantic Summer Fiction 2007 issue states it thus:
A number of the top programs told me, “We had (twelve) slots,” let’s say, for fiction writers. “We offered our first twelve, and all twelve accepted and came here.” I heard that from a number of the top programs, and I know that a lot of students apply to all of these programs. So it suggests to me that each of these programs, either consciously or unconsciously, is choosing certain types of writers.