According to this Salon article, Clark claims that a Democratic ticket starring him as vice-president would not be able to win the election.
While I understand his reasons for saying this - if he was gung ho about the possibility this early, it would not bode well for his campaign's morale - I am still a bit disappointed. If Clark does not win the nomination, the best thing for the Democratic Party would be a Dean-Clark ticket, especially if the primaries end with a Dean-Clark head-to-head grudgematch. As the aforementioned Salon article mentions, Clark has a lot of grassroots support.
Of course, so does Dean. Which makes it just as imperative that if Clark wins the nomination, he should select Dean as vice-president.