Cook Political Report (CPR) does a good job of evaluating races. It has put 6 senate races into the Toss up section. Of the six 5 are currently repubs and 1 is a dem. That is Obama's old seat. The reason is b/c Burris has lost several state wide elections and unless something dramatically changes, he will probably lost that seat over the next two years (when the seat will be available for election).
The repub states are Missouri, Florida, Ohio, Louisiana (b/c of his brothel problems), and Kentucky. Besides Louisiana, the other 4 are retiring.
It is only going to get worse for the repubs over the next 2 years.