Cook lists seven Democratic Senate seats as being tossups: Begich (Alaska), Pryor (Arkansas), Udall (CO), Harkin (Iowa), Landreiu (Louisiana), Levin (Michigan) and Hagan (North Carolina).
Recent polls have shown Republican challengers moving ahead in Alaska and Colorado, two states where they had previously trailed. I would disagree with Cook on Michigan and North Carolina. They have polled solidly for the Democrats, albeit not by huge numbers, all summer. I would put them in the lean Democratic column instead of in the tossup.
One seat that Cook lists in the lean Democratic column is Shaheen of New Hampshire. A poll just released show former Massachusetts Senator Scott Brown drawing even in that state. I think that state may be moving to the tossup category.
As far as the Democrats picking up Republican seats, Kentucky is still listed as a tossup state. But Republican Majority Leader Mitch McConnell has consistently enjoyed a small lead in virtually every poll. The race shows no sign of tightening. I think it should be moved to the lean Republican category.
The Democrats probably best shot at a Republican seat is in Georgia where the polling has been all over the place, though generally in favor of the Republican Perdue over Democrat Nunn in the battle for an open seat. Also, the Democrats have a shot at knocking off Kansas Senator Pat Roberts who is in a neck-and-neck competition with a left-of-center independent who is likely to caucus with the Democrats.
I would put it at about 65% that the Republicans will gain the net six seats needed for the majority. If pushed to make individual state predictions, I would guess Republicans will pick up seats in Montana, South Dakota, West Virgina, Louisiana, Arkansas, Iowa and Alaska, the seven seats necessary for the majority. I doubt that, at the end of the day, the Democrats can pull off upsets in Kentucky, Georgia or Kansas.