This makes Joe Lieberman the most powerful man in Washington. He can threaten to cross over if the Dems try ANYTHING he doesn't like.
If he plays that card too often and the Dems pick up any more seats in 2008 (almost certain -- the GOP is defending 21 seats, the Dems are only defending 12) he'll learn the ancient lesson that payback is a bitch.
Ergo, I expect him to hold that "nuclear option" in reserve for emergency use only.