At least he's refusing to rule it out:

Senator Joseph I. Lieberman of Connecticut said yesterday that he will caucus with Senate Democrats in the new Congress, but he would not rule out switching to the Republican caucus if he starts to feel uncomfortable among Democrats.

Democrats have no right to expect blind loyalty from Lieberman after the way they treated him this last election cycle. The fever swamp left, and its attempt to get Lieberman defeated and Ned Lamont elected, was defeated in that endeavor and may yet experience the further consequence of an independant Lieberman moving to the Republican side of the aisle.