i'd say most summer research programs have a 5-15% acceptance rate depending on the program. places like UCSF are probably around 5%, and placed like Baylor/Mayo are around 10%. this is based on the numbers they gave me, but i'm sure there are programs out there that are much easier.
also, your year in college is important, too. last year, i was rejected from a couple because they said they wanted to give the experience to juniors who wouldn't be able to do research the following summer. they also rarely give positions out to freshmen. i think you don't really need prior research to get in to most programs... you just need a great essay on why you want the opportunity.