Here's a question: If Lieberman switched parties tomorrow, what would actually happen in the Senate? When Jim Jeffords defected in 2001, Senate Democrats got to elect Tom Daschle Majority Leader and assumed control of all the committees. But that was only because earlier in the year Daschle and Trent Lott had explicitly agreed that whichever party won majority control of the Senate would also get the leadership, committee chairs, etc. If I'm not mistaken, they'd made the deal back when the presidential race--and with it, control of the 50-50 Senate--was still up in the air.
Now, as far as I know, there's no such deal in place today. The leadership and committee positions have already been set. Any attempt to reorganize the chamber would be subject to a filibuster. Even if Lieberman does defect, the GOP won't really benefit. So I don't see what incentive Lieberman has to do so--especially since Dems will probably pick up seats in 2008, in which case he'll really be screwed.