Lieberman has now become one of the most powerful members of the Senate and he is smart to keep the Democrats on the edge, not getting too comfortable with what he will do:

Sen. Joe Lieberman on Sunday repeated his pledge to caucus with Senate Democrats when the 110th Congress convenes in January, but refused to slam the door on possibly moving to the Republican side of the aisle.
Asked on NBC’s “Meet the Press” if he might follow the example of Sen. Jim Jeffords of Vermont, who left the Republicans in 2001 and became an independent, ending Republican control of the U.S. Senate, Lieberman refused to discount the possibility.
“I’m not ruling it out but I hope I don’t get to that point,” he said. “And I must say — and with all respect to the Republicans who supported me in Connecticut — nobody ever said, ‘We’re doing this because we want you to switch over. We want you to do what you think is right and good for our state and country,’ and I appreciate that.

I’m glad he’s got this power because he can put a stop to this kind of thing with it:

Democrats, who won control of the U.S. Congress, said on Sunday they will push for a phased withdrawal of American troops from Iraq to begin in four to six months, but the White House cautioned against fixing timetables.
“First order of business is to change the direction of Iraq policy,” said Sen. Carl Levin, a Michigan Democrat who is expected to be chairman of the Senate Armed Services Committee in the new Congress.
Democrats will press President George W. Bush’s administration to tell the Iraqi government that U.S. presence was “not open-ended, and that, as a matter of fact, we need to begin a phased redeployment of forces from Iraq in four to six months,” Levin said on ABC’s “This Week” program.

More than any other Senator he can say that he was elected because of his stance on the war (that was the one distinctive between the candidates) and that his constituents don’t want the troops removed until Iraq is ready.
But though he may toy with the idea, I bet he’ll stay with the Democrats. I actually think it’s a good for the Democrats to have someone like Lieberman, he brings a breath of sanity to the party, which is desperately needed.
And BTW, the last time I checked it was the President who decided when the troops came home, not the Congress. Now Congress could stop the provision of funds to the troops, but will they really do that? Are they really that sure that the American people are sick of the war enough to stop funding our men and women who are in a foreign land fighting and dying for a war that they approved?