I'm not sure anyone should find this surprising. Lieberman is not exactly enamored with the Democratic Party after primary voters dumped him 2006, and the Party is likewise not thrilled at Lieberman's increasingly reactionary conservative stances on foreign policy. The odd thing, in a sense, is that Lieberman isn't that conservative on other issues. He's certainly not a liberal icon, but outside of foreign policy and business regulation, Lieberman is pretty well in line with the rest of his party. But over the past several years, it's become increasingly clear that a hawkish foreign policy agenda is his political raison d'être. The endorsement of McCain is the culmination of that shift.
So will it matter? Back to CNN:
This endorsement could help emphasize McCain's national security standing, show he is able to work across party lines, and perhaps help persuade independent voters in New Hampshire to support his presidential bid.
Possibly true -- I think in New Hampshire, particularly, Lieberman might have some sway on the independent voters (who can vote either in the GOP or Democratic primary). But by and large, I'm not convinced it will have much impact. A candidate needs more than New Hampshire to win a nomination, and I don't see Lieberman's influence extending beyond that neighboring state's quirky political climate. Lieberman isn't an influential Democrat anymore -- in fact, he isn't a Democrat at all anymore. He gets to be this year's Zell Miller, and I don't really think ol' Zell ultimately had a big impact in 2004.