If Apple really had a problem with it, they'd block it from their machines. Since this increases interest and use of the machines, they're not gonna do it.
They've blocked MySpace, but not Facebook. I suppose it's because they have a (relatively) good relationship with Facebook, what with the web app on their commercials and all.
There are three stores in my area, two are in suburbia with suburban high school girls obnoxiously using PhotoBooth before realising MySpace is blocked, and the third is in the center of town in a high end shopping district, but it is always jammed with about two or three hundred people in it and too few employees to service them all.
I saw the manager checking out the store yesterday, and I said to the person I was with, pretending I didn't know the manager was there, that I can never find a mac specialist to talk to.
Oh, and the other thing about these obnoxious girls is that I go to school with a lot of them, and obviously friends with them on Facebook, and their profile photos they took always look so tacky because you can clearly see they took they photos in an Apple store.