Cook Political Report - Early tracking US 2012 senate races indicate that the Republicans have a good chance to get a majority. Currently there are 51 Democrats, 47 Republicans and 2 Independents.

33 Senate seats will up for election, 23 Democrat and 10 Republican.
9 are solidly Republican and 9 are toss up seats.
If the Republicans get 4 or 5 toss up seats then the Senate would become 51 Republican, 49 Democrat/Independent.

The House will stay a Republican majority with some chance for an increased majority.