This is mostly my conjecture. I've based it on facts, recent elections and some of the things I've read.
As of now, there are 53 Democratic/Democratic-caucusing Senators. In 2012, there will be 23 Democratic and 10 Republican Senate seats up for grabs. In addition, many of the Democrats are in red states. Here's how I see the probabilities:
Democrats certain to lose
Kent Conrads old seat after he retired
Ben Nelson of Nebraska
Democrats likely to lose
John Tester of Montana
Claire McCaskill of Missouri
Democrats in trouble
Debbie Stabenow of Michigan
Democrats in Tossup Races
Bill Nelson of Florida
Sherrod Brown of Ohio
Republicans in Trouble
John Ensign of Nevada (although he could get taken out in a Republican primary anyway)
I'd have to do some research on how Tester is faring, but for now I think that he and McCaskill are in trouble and likely to lose. I think it will be hard for McCaskill and Tester to run away from their votes on the stimulus, the bailouts and healthcare. They may not get away, and thus they may get taken down in those red states. So, not counting tossups, Dems are -4, which puts them at 49 seats against 51 Republicans. Democrats will need to face low quality Tea Party candidates like Sharron Angle and Christine O'Donnell in order to be competitive. This could help Democrats a lot in all of these races in addition to Tea Partiers launching primary campaigns against established Republicans in Maine (Olympia Snowe) and Indiana (Dick Lugar). For the record, while many of the GOP Senators are scumbags, I think it is somewhat tragic that in order for Democrats to keep control of the Senate, respectable senators like Lugar would have to be defeated.
We could be looking at a second Bill Clinton scenario of 1996 with a Democrat in the White House and Republicans owning the Congress, or with a very low chance, the Democrats could take the House back, but not the Senate, or the Republicans could run the whole board and take everything back.
The Democrats taking back the House could be unlikely though, as Obama's support among working class whites and independents may have dipped too low for another wave election.