Nate Silver is a poll analyst at site FiveThirtyEight. He is unquestionably a liberal, but many consider him an unbiased poll analyst. He is predicting the democrats will only control 53 or 54 seats after the 2010 election. According to his analysis, there is a slight chance Republicans could gain control.
...there's about a 6-7 percent chance that Republicans could actually take control of the Senate, and another 6 percent chance or so that they could wind up with a 50-50 split. On the other hand, there's still a 7-8 percent chance that the Democrats could regain their 60th seat if the national environment shifts back in their direction.