Nate Silver's two cents:
Democrats if they are expecting to do better in 2012 than they did this time around might actually be pleased that elections have become so strongly aligned to partisan orientation. They now have just 12 seats in which Mr. Obama won a minority of the vote to defend whereas Republicans have 55 where he took the the majority instead. So if there is even a fairly modest shift back to Democrats in 2012, and the shift is again fairly uniform, they could be in a position to achieve quite a few gains.
Or, if the economy improves and having facilitated a more even balance of power in Washington the electorate becomes somewhat less angsty, the incumbent advantage could become stronger again, and the gains that Republicans made could prove to be relatively “sticky” as they were, for instance, after 1994.