Nate Silver has run his new-fangled Senate Predictatron forecasting model and found that the most likely outcome is a net 6-7 seat loss for Senate Democrats. Moreover:
The Democrats now have an approximately 20 percent chance of losing 10 or more seats in the Senate, according to the model, which would cost them control of the chamber unless Gov. Charlie Crist of Florida, who is running for the Senate as an independent, both wins his race and decides to caucus with them.
In addition, there is an 11 percent chance that Democrats will lose a total of nine seats, which would leave them with 50 votes, making them vulnerable to a defection to the Republican Party by a centrist like Joseph I. Lieberman of Connecticut or Ben Nelson of Nebraska.
Silver gives a broad overview of his methodology, and fine-grained details about particular races, here.