Let's review the story of Joe Lieberman. He lost the 2006 Connecticut Democratic Senate primary to Ned Lamont and then beat Lamont and an irrelevant Republican in the general election while running as an independent and being supported almost entirely by Republicans despite stating publicly that he would caucus with the Democrats (i.e., vote for Harry Reid for Majority Leader). This was a very important vote, because with the Senate having 49 Dems, 49 Republicans and 2 independents (Lieberman and Vermont's Bernie Sanders, who also caucuses w/ the Dems) meant that the Dems controlled the Senate and all the committee chairs. If Lieberman had caucused with the Republicans and voted for Mitch McConnell for Majority Leader, then it would've been a 50-50 split which would have been broken by Dick Cheney.
Then, as you might be aware, despite being a pretty standard, down-the-line Democrat on all issues outside of Iraq, he decided to publicly (very publicly, it must be said) support McCain for President, going so far as to speak at the Republican convention in September. Democrats were pissed, but, because Democratic Senate leadership and committee chairmen needed Lieberman's vote in order to remain Senate leadership and committee chairmen.
Now, with the new Senate based on Election Day looking like 55 Dems, 2 Independents, 40 Republicans and 3 still undecided (though all leaning, to varying degrees, toward the Republicans), it looks unlikely that Lieberman's caucus vote will matter very much. Being the 51st vote in a 100-vote body is a much stronger position than being the 57th, so now Harry Reid is ratcheting up the pressure on Lieberman to give up his committee chairmanship; he currently leads the Homeland Security committee. The likely outcome of stripping Lieberman of his chairmanship would be that he would caucus with the Republicans, making it 56-44 (assuming Franken, Begich and Martin fail to come from behind in any of their undecided races).
There are two reasons why stripping Lieberman of his position, effectively kicking him out of the caucus, is a terrible idea. The most obvious is that it's still possible that the Democrats will reach a filibuster-proof 60 seats, if they should manage to run the table in the three undecided races. It's also possible that, with 19 Republican seats and 15 Democratic seats up for election in 2010, that the Democrats will pick up the three needed seats in that election. Booting Lieberman out would mean they'd have to win 4.
But more importantly, it is utterly ridiculous for them to boot him out now, as opposed to the day after he spoke at the Republican convention. Stripping him only after his vote is no longer necessary to keep you in power is spineless, and far from a new sort of politics that President-elect Obama campaigned on. One of the few things that would make me respect him even more than I already do would be if he were to, as a matter of principle, instruct Reid to back off of Lieberman.