First, he didn’t explicitly say “I’d vote for Obama over Romney”. What he was doing was saying what a lot of people here have said — that Romney might not be that big a contrast to Obama.
GIven that this isn’t a policy issue, or something where if you follow his words you’ll do the wrong thing (it was meant to stop people from voting for Romney, and that is the RIGHT thing), I don’t see what the point is of making a big deal out of the statement. He’s already pointed out that he won’t vote for Obama over Romney. As others have noted, he endorsed Romney in 2008 against Huckabee and McCain, so does it make sense he would vote for Obama over Romney now?