This puts Senator Obama into an amusing position. He can be silent on this, which is my expectation (well, okay, he might make a low-key comment that he's already stated his opposition to the measure as "divisive" or somesuch, but I doubt it), and remind all the gayfolk that he's opposed to actual legal equality, or he can make a clear and relatively strong statement against the initiative, and unleash the horde of "Obama's a faggot lover" commercials I rather suspect the fundamentalists of the GOP already have lined up.
Now as I've noted before, there's precious little support in the "progressive community" for Senator Obama sticking his next out for queers - quite the opposite, in fact, go read that post if you haven't already - so I don't think it'll hurt him with the bulk of even his pro-marriage supporters. Given that, it's not really much of a box - which side to break is pretty trivial. But it still illustrates the kind of box someone can build for you when you try to have things both ways.
(BTW, I do note with quite a bit of surprise that Bill Clinton recorded a robocall for the No on 8 side. Given the disaster of Don't Ask/Don't Tell, his trumpeting - at the time - of DOMA, and his vicious advice to John Kerry to endorse anti-marriage initiatives in 2004, I'm surprised - and not even sure how I feel about it.)