Sean Trende surveys the 2012 political landscape:
Right now Republicans hold 47 of the 100 seats; if they win the presidency, they would need to pick up three more seats to have a majority. As of today, they are favored to pick up two Democratic seats. Two Republican seats are tossups, along with six Democratic seats. If we assume Democrats pick off one of the two Republican tossups, and that Republican pick off half of the Democratic tossups, Republicans would have a 51-49 majority.
Of course, Senate tossups have tended to break hard toward one party or the other, which suggests that the Republicans have much more upside than Democrats. If Republicans were to sweep the tossups, they would have a 55-45 majority, tying them for their largest advantage since 1928. With a large number of Democrats up for re-election in 2014, many of whom occupy seats in red states, that would probably give Republicans working control of the chamber.
For right now, though, it is safe to say that Republicans are favored to take control of the Senate.
Read the whole thing for Trende's state-by-state analysis.