Not yet.
No seasoning. He's a first-rate orator, and an original thinker, but only a first-term Senator. He needs more time at the national level, just for experience.
He's playing coy right now, because so many people have brought his name up for 2008, but I think he's got the tone right: "I'm thinking about how I can be useful to my country, and I'm not ruling out higher office." Well, no, of course not. But he has to know he hasn't even finished his first term yet.
He's young. Leave him in the bottle a little longer.