If Leiberman switches parties, again, nothing happens. You heard me right, absolutely nothing.
From "Political Insider"
"With Sen. Joe Lieberman (I-CT) publicly stating he'd consider becoming a Republican if Democrats block new funding for the Iraq War, many Democrats worry that control of the Senate hangs in the balance. However, their fears are unfounded. Many think back to 2001 when former Sen. Jim Jeffords (I-VT) began caucusing with Democrats instead of Republicans, taking control of the Senate out of GOP hands. However, the two situations - though outwardly similar - contain one important difference.
If Lieberman were to caucus with the Republicans, they would still not take full control of the Senate, despite Vice President Dick Cheney's ability to break 50-50 ties. This is because of a little-known Senate organizing resolution, passed in January, which gives Democrats control of the Senate and committee chairmanships until the beginning of the 111th Congress.
What's the difference between now and 2001? A small but important distinction. When the 107th Congress was convened on January 3, 2001, Al Gore was still the Vice President and would be for another two-and-a-half weeks. Therefore, because of the Senate's 50-50 tie, Democrats had nominal control of the chamber when the organizing resolution came to a vote. With Dick Cheney soon to come in, however, Democrats allowed Republicans to control the Senate in return for a provision on the organizing resolution that allowed for a reorganization of the chamber if any member should switch parties, which Jeffords did five months later. There was no such clause in the current Senate's organizing resolution."