Other than ideology, inertia, and nostalgia for his career as a Democrat, of course, I don't see any reason why he shouldn't. From a purely tactical perspective, it makes a lot of sense. If Lieberman joins the GOP, he gives them control of the Senate (with Dick Cheney's vote), which could marginally help John McCain's chances of winning this fall (the GOP would determine which measures came up for votes, etc.), which is the only way Lieberman isn't toast next year.
You might argue that Lieberman wouldn't want to piss off Senate Democrats who will in all likelihood have a comfortable majority in January, and will be in a position to exact revenge if he's still in the chamber. But, according to this amusing Mark Leibovich piece in today's Times, he's already thoroughly pissed them off. So I don't expect many warm feelings toward Lieberman if he sticks it out as an "independent Democrat" between now and then. (My sense is that the bad feelings run so high Senate Dems won't be restrained by the possibility that they'll need his vote to break a filibuster.)
I guess the concern is that the backlash against giving the GOP control of the Senate via party-switch could energize Democratic voters and help the party raise more money and gain more seats. But, from Lieberman's perspective, that's probably an acceptable risk. As I say, he's pretty much screwed if McCain doesn't win and appoint him Secretary of Defense or something.
Update: Ah, ask and you shall receive... Brad Plumer supplies the answer:
Even if Lieberman jumped ship, Democrats would still keep control of the Senate. The only reason the Senate flipped when Jeffords defected in 2001 was that Daschle and Lott had earlier worked out a power-sharing agreement handing control of the Senate to whoever had 51 votes. This year, there's no such agreement, so the GOP would have to pass a new resolution reorganizing the Senate, something the Dems could easily filibuster.