It was a great interview. Really, really great- this was the kind of thing that could really sway opinions (assuming enough people watch). Jon was surprisingly quiet and low key, asking fairly open (yet probing) questions and keeping the jokes to a minimum, and Clark's stories and comments came out as pretty natural, despite having surely been pre-rehearsed. It dealt with the "you have no political experience" question very well, and Clark seemed quite comfortable, more so than many of his other guests.
I'm thinking that Jon is a closet Clark supporter more and more, especially considering how relatively unsupportive he's been of Dean, and the fact that Dean is the last major contender (aside from Kerry) who hasn't appeared on the show. He could simply be working up to Dean, or it may be the Dean campaign that has been rebuffing Jon's overtures. Still, after this interview and the reality that it's turning into a Clark/Dean/Gephardt race, it may be safe to assume that there's a real reason they haven't brought Dean on.