My thoughts....
Potential Republican Pickups (in order of likelihood):
1. Alaska - A lot of Republican heavyweights are exploring the idea of challenging Begich. Though there's nothing particularly offensive about the man or his tenure as Senator, any competent and well-backed Republican should be able to take him out. This assumes, of course, that the Tea Party doesn't gum up the works and leave the Republicans with a gawd-awful candidate like Joe Miller.
Absolutely crazy thought I had; but...Sarah Palin?!?
After leaving Alaska in mid-term to pursue the vice-presidency, and then a career in the right-wing media world, no one in Alaska is prepared to entrust her ever again with public office.