Scott Brown is up in 2012. He's probably one of the more vunerable ones.
Kyl, Lugar, Wicker, Corker, Hutchinson, and Barrasso are all pretty safe.
Ensign has some problems, and Hatch, if he doesn't retire, could face a primary challenge.
And half of Obama's "Comfort Girls", Olympia Snowe, might face a challenge, but probably not.