It's funny, but I wasn't expecting Kerry to be the pretty much de-facto leader, if not candidate. Clark, Edwards, Dean, sure, but not Kerry. He was so down in the polls and so relatively unknown that the only plus he had, "electability", was pretty much out the window.
Two elections later, and provided he doesn't screw up badly, he's got it in the bag. Clark's hyper-focus on New Hampshire went nowhere, Edwards looks more and more like a prospective VP instead of a president, Dean is trapped in second place with the "unelectable" problem hanging around his neck like a millstone, and Lieberman is circling the drain. Kerry's problem was that he had no momentum or money, and it looks like neither are an issue at this point. (For the latter, look here). It seems like everybody is closing around him- Dems and media both. Is he as electable as he seems? Guess we'll find out in the General.
(Unless, of course, the aforementioned screwup happens, or Edwards really does come roaring out of the South.)
So, if he isn't going to get a chance to be the candidate, then what did the Dean campaign accomplish? I'm of the same opinion that a lot of commentators have been (over at Kos, for example) that he lit a fire under the Democrats and changed the terms of the debate. Any smart Democrat is going to be acutely aware that even if Kerry is the candidate, the message that Dean brought to bear isn't going anywhere- that the base is really ticked off and isn't going to stand for another 2002. He may, I think, be the Democrats' Goldwater, where the sort of realization of collective identity that conservatives went through after that campaign will come to pass again I still think he could have been their Reagan, too, but Democrats don't appear to be ready for fiercely partisan candidates.
In any case, the task now is to figure out how to ensure that Kerry doesn't get pigeonholed as another Gore, and to make sure that the activist base doesn't give up and go home because their favorite candidate probably isn't going to be the nominee. If the base stays home, Bush stays too.