First, let's look at the Dean meltdown itself. Here's the point at which Kerry pulled ahead in Iowa (January 14th). It really is an epic flameout when you look at the numbers; Dean went from 10-point frontrunner to 5-point underdog in 1 week. That being said, Dean was only the favorite in Iowa for a short time, having pulled ahead of Gephardt sometime in November or December.
Now, check this out, the NH numbers according to ARG around the time of the Iowa Caucus. On January 3, Dean has 39% support to Kerry's 14%. By the 17th, that number has dipped to 28%, nearly the entirety of the difference flowing to Kerry over the next couple of days to 19% (after a dip to 10%). Closer, but Dean's still clearly ahead 28-19%. Dean's numbers hold steady until the 19th, the day of the Iowa Caucus, after which they drop a breathtaking 12 points in 5 days, with nearly the entire difference going to Kerry. By the 24th, Kerry led Dean 38-16%.
So yeah, Iowa matters. It wouldn't be the first time the cornfed kingmakers lifted a moribund campaign to instant anointed frontrunner status. And it is easy to forget how moribund the Kerry campaign appeared: I distinctly remember Jay Leno telling a joke in late 2003 about how the Kerry campaign was "on life support."
However, looking at national polls, Dean even in his heyday never commanded nearly the numbers of Clinton. This particular poll, from Dec. 2003, only gives Dean 23% to everyone else's 10%-- over 50% were still undecided. Comparatively, Clinton currently polls in the forties, and one poll had her poking over 50%.
Furthermore, the '04 candidates were far less charitable to the front-runner than Obama and Edwards are to Clinton. Dean joked about how he knew he was ahead because he was picking so much buckshot from his backside. He was treated as an outsider from the very beginning, a rabble-rousing yokel from out of town who just didn't know how things are done. Kerry and Lieberman let him have it frequently, and Gephardt burned his entire warchest in a knock-down drag-out in Iowa that drove up Dean's negatives in that state and may have won the caucus for Kerry.
In fact, I have heard rumors from people on the ground in Iowa at the time that Gephardt saw the writing on the wall and intentionally immolated himself killing Dean for Kerry's sake. It would explain this rather strange moment in the middle of the campaign:
In December, Mr. Kerry, in an unguarded moment, did not hesitate when asked whom he would support if he were not in the race: ''Gephardt.'' Similarly, Mr. Gephardt did not hesitate when he was eliminated from the Democratic race the night of the Iowa caucuses to say whom he would support.
Perhaps it is still early, but Clinton has no Gephardt at this point willing to empty his clip on her, no DLC working to undermine her, and no Kerry or Lieberman legitimizing her attackers.
Yes, it is true that Dean didn't start his descent until January of '04, and yes it is true that he dropped some 25 points over 3 weeks, and a similar drop from Hillary would be enough for Obama or Edwards to run away with it. However, Dean was working under far more adverse circumstances than Clinton and never had a lead as large as hers. So I'm not saying such a comeback is impossible in '08; I just don't see a mass exodus away from Hillary like what happened to Dean.
Then again, I didn't see USC dropping one to Stanford, either, so what do I know?