Not sure, but he recently (since the election) refused to rule it out. My biggest worry about him is that he lacks the charisma to win a campaign; luckily, that is what the primary season is for.
Well, in this day and age, it seems that a candidate without charisma is going to be lost in the glow of the ones who have it.
Where would O be without charisma? Probably still community-organizing.