If Dean doesn't win the Democratic nomination, what political role do you see him playing in the future? Of course, this depends on how the presidential election plays out, so I'm listing various potential scenarios below:
Dean doesn't get the nomination, becomes Clark's VP, Clark wins: I think Clark would be wise to give VP Dean a wide-ranging domestic portfolio and make him the administration's dealmaker in the Congress. (I think he would be quite good at this, actually, since he is by all accounts a savvy politician.) He could also be the designated spokesperson for raising difficult issues, which would let Clark sit back and be "presidential."
Dean doesn't get nomination, is not chosen as Clark's VP, Clark wins: Would Dean be offered a cabinet post or would he be considered damaged goods at this point? Or would his fundraising and organizing prowess still be acknowledged by the Dem insiders? My guess is Clark would want to offer him a cabinet position (perhaps hint at this in the GE campaign) in order to keep at least some hardcore Dean supporters in the game.
Dean doesn't get nomination, is chosen as Clark VP, Clark loses: I think this scenario carries greater liability than the one below, but Dean would still have established name recognition for himself for a future run.
Dean doesn't get nomination, not chosen as Clark VP, Clark loses: Dean would not have been bloodied in the 2004 general, could claim vindication, and potentially become a player in the 2008 race. If Hillary decides to run in 2008, I believe Dean would have a difficult time winning the nomination against her. If she decides not to, he should have enough residual grassroots loyalty to launch another campaign.
The interesting thing is that Dean has never lost an election, so no one knows how he would react to a defeat, but I think he would be foolish to give up the national profile he has staked out for himself. The Democratic Party needs all the high-energy, creative individuals on its side that it can get.