As a conservative, I'm hopeful about 2012 in the Senate, but we have no idea what the conditions will be in 2012. Just look at how things changed in the last two years. We can't discount the R presidential nominee either. If it's a dud like Palin or Huckabee, many of these guys might not try for the seats. If Jeb Bush runs, well that's another story.
Unfortunately, the bench isn't deep for Republicans thanks to 2006 and 2008. ND, MO, MT, VA, OH, FL, NE, and even HA look like possibilities for Republicans, but who runs? Rehberg in MT, Lingle in HA, and then who else? That's the problem I see for Republicans. Throw some names at me for these states.
As for Snowe and Brown, they're not going to be primaried or retire. Snowe will win and Brown's got at least a puncher's chance. Certainly don't see any Republican retirements right now. At least not ones that would make competitive races. We went through that in 2006.
ND: Lt. Governor or Attorney General or something? I think they are all GOP
MO: one of the GOP congressmen or the state auditor
OH: No great candidates come to mind unless Kasich wants a change, but Brown is so liberal that a rando might win here
VA: Bolling or Allen or Randy Forbes (the AG Cuccinelli might try, but he would likely lose to Webb)
FL: Jeb Bush? All of the statewide offices are held by the GOP, so the possibilities are virtually endless
NE: Heineman (the governor with 70% approval)- this is as over as ND 2010 if he runs
Also, Capito could try to take out Manchin in WV, hoping that Obama being on the ballot will hurt Manchin.
This is how much Obama hurt the Democrat in the last statewide race in West Virginia:
http://en.wikipedia.org/wiki/United_States_Senate_election_in_West_Virginia,_2008If Capito wanted that seat, this was the year to do it. She missed her chance, and she'll have to settle for the job that will soon be vacated by Joe Manchin.