It's way too early for any sort of electoral predictions on the national level given that the passage of health care reform and any positive signs in the economy can dramatically change the playing field, but, since Nate Silver released his 2010 Senate projections for September, I figured why not?
Assuming the top 10 races are the most vunerable, we see that five of the seats are held by Democrats and five are held by Republicans. I'd flip-flop Delaware with North Carolina, as I just don't think it's feasible for Democrats to pick up a Republican seat in a Southern state in the mid-terms, and you've got six defensive seats for Democrats and four for Republicans (and that makes sense given the existing 60-40 breakdown in the Senate).
As a result, the 10 seats most likely to flip one way or another are ...
1. Missouri (Republican-held)
2. Nevada (Democrat-held)
3. Ohio (Republican-held)
4. Connecticut (Democrat-held)
5. Colorado (Democrat-held)
6. New Hampshire (Republican-held)
7. Kentucky (Republican-held)
8. Arkansas (Democrat-held)
9. Illinois (Democrat-held)
10. Delaware (Democrat-held)
A couple of quick observations before digging deeper. First, the Senate is going to remain in Democratic hands. There's just no logical way Republicans can flip 10 currently held Democratic seats, and, though it's arguably feasible, I have doubts they can flip three or more of these seats. Second, there's a very good chance Democrats maintain a significant advantage in the Senate, though it seems probable they'll lose the filibuster (thought, as of yet, it has yielded any results for them and been a fundraising boon for the Republicans).
As of now, I'd assume both Missouri and Ohio are going to be Democratic pickups. They are two states that are trending sharply progressive, and they both feature popular candidates on the Democratic side. The polls show some tightening right now, but the vitality of the economy will play a key role here.
Likewise, New Hampshire - despite the candidacy of attorney general Kelly Ayotte on the Republican side - makes a more friendly spot for progressives. New England has become progressive territory, the president is popular here and State Sen. Paul Hodes has been campaigning for some time now. Ayotte's presence could swing it another way, but it's a clear toss-up right now ... which, long-term, is good for Democrats in a Republican-held seat.
Nevada and Connecticut, however, look problematic. I don't see how Sen. Harry Reid keeps his job back home, despite his massive fundraising war chest. He's one of the most visible Democrats in the country, and that means Republicans are going to throw everything they've got at him. He's already lagging in the polls to challenges with much lower name recognition, and that's not good.
In Connecticut, Sen. Christopher Dodd is hurt by his ties to the financial markets. That may evaporate by 2010, but it's hurt his favorables. One positive thing going for him, however, is that he's running against a former staffer for President George W. Bush and, even though it'll be two years removed from his presidency, New Englanders don't particularly care for the man. I still say Dodd has an uphill battle, and I can see him losing this one.
In addition, I think Colorado goes from blue to red this cycle, largely because of the absurd selection of Michael Bennett as Ken Salazar's replacement. The state has soured on President Obama in recent months, and that's going to hurt Bennett.
I see Republicans holding Kentucky (though if Rand Paul, son of Rep. Ron Paul, wins the GOP primary, Democrats have a legitimate shot) and taking Arkansas away from Democrats. In addition, I think Delaware is a likely pickup option for them given the popularity of Rep. Mike Castle, even though it's a staunchly Democratic state. If Beau Biden runs for it, his name recognition could help him, but Castle figures to be the man to beat.
Illinois, in the end, stays with Democrats. Rep. Mark Kirk has already made a couple of key missteps and moved away from his centrist positions to cater to the Republican base. Democrats have a strong stable of candidates who figure to hold the seat.
In the end, that's four pickups for Republicans against two for Democrats.
Ultimately what's likely going to happen is Congress is going to return to a political environment that more closely resembles the 2006 congressional landscape, with Democrats holding more narrow margins in both chambers.