Chait thinks Lieberman is bitter, but that his filibuster threat will prove irrelevant:
My guess is that ultimately he'll vote for reform, but he'll do so because the Democrats will scale back their plan and win over Olympia Snowe, making Lieberman's opposition academic. Lieberman won't join a futile filibuster, but if he has the chance to stick in the knife and kill health care reform, I think he'd probably jump at the chance.
Marc's read:
[T]he final bill, post-conference, is going to look a bit different from the reconciled Senate bill. Lieberman is giving himself the power to influence the final bill. I doubt that the Senate leadership is going to press him too hard right now, preferring to see if he can be accommodated in the final debate.