I find this interesting. Ross Douthat's point is well-taken: it doesn't make a whole lot of sense for Democrats to celebrate the leftward drift our party has taken while excoriating Joe Lieberman for basically saying that. But it's understandable. For one thing, it's Joe Lieberman, one of the most shifty and loathsome pols in our current polity. I suppose that's not fair: Lieberman is an unflinching advocate of war (that's a principle), and he's still generally liberal on social issues, though he's "centrist" on many of them. He's not exactly followed through on his promises of remaining loyal to the Democrats, but the people of Connecticut should have known better in 2006 than to vote for someone who chooses policy positions by pique.
The other factor is that Lieberman's messing with the JFK aura. Usually, that's a no-no. Matt Yglesias, though, tends to take a dim view of John Kennedy, and I think he's about right here. Kennedy was much more of a hawk than some might remember--just look at the provocative military action his team undertook with respect to Cuba, Berlin, and Vietnam. Now, Kennedy wasn't a bad president as Yglesias likes to maintain (though he was hardly a great one, and had he lived he probably would have lost to Barry Goldwater in 1964, as his domestic agenda had stalled in Congress) but at some point I think it would be helpful for Democrats to admit that their admiration for Kennedy isn't really based on Kennedy's policies, which were avowedly centrist for the time, but rather on the entirely manufactured romanticism of Camelot. Now, Bobby Kennedy was a true liberal, but JFK wasn't. It's similar to the right's Reagan worship, but worse in my opinion because Reagan was actually a conservative and he did actually lower taxes and take on the Soviet Union. The JFK love is nothing more than a bit of nostalgia for the Kennedy-era idealism, generally.
