Nelson is quite unlikely to switch. He is probably going to retire in 2012 anyway and doesn't seem overly concerned about his re-election prospects. Lieberman could very easily switch, however. If the Senate gets to exactly 50-50 after the elections, expect him to play both sides against each other to see where he can get the better deal. Also, watch Snowe closely. If the Republicans win control by exactly 51-49, I could see here defecting and throwing the Senate back to the D's. She will be under constant threat of a Tea Party primary in 2012, especially if LePage wins the ME governor's race this fall.
I don't believe Snowe will do such a thing because she has been very closely tied with the ME GOP. However, politicians do surprising things when their careers are in danger. We'll see.