One of the more amazing things about the Democratic presidential nomination race is that, even though so many party insiders held back before the primaries and just waited for voters to figure it out, the party insiders will end up deciding this race after all. If you play with Slate's delegate calculator you'll note that it's pretty much impossible for either Clinton or Obama to win this thing without the aid of superdelegates. The primaries are now playing the role they did prior to 1972 -- they're beauty contests in which party insiders get to evaluate the candidates' relative strengths and weaknesses before actually deciding on the nominee.
Of course, it's likely that Obama will be the leader in pledged delegates by the time the primaries and caucuses are over, but only by 100 delegates or so. He won't clinch it. So won't the superdelegates just go with the pledged delegate leader? Not necessarily.
Josh Marshall spins this out nicely. In short, this last week was the first time that Obama experienced some actually bad press, and it was also the first time the Clinton campaign went seriously negative on him. And all this stuff seemed to have an effect, breaking his winning streak and costing him 3 out of 4 races yesterday. So the impression insiders might be getting is that Obama has a glass jaw, and they don't want to put such a fragile candidate up against McCain (who will be a lot nastier that Hillary) in the fall.
I imagine Obama can quell some of these concerns with big wins in Wyoming and Mississippi in the next week, and maybe he'll start going more negative on Clinton (although it's amazing how far he's gotten without doing so). But this scenario, if improbable, is far from impossible. Of course, then the superdelegates will have to be concerned about what this would do to the Democratic Party. All these African Americans and young people who have been volunteering and voting in record numbers over the past few months... how will they feel if their candidate, who won more states and more pledged delegates by greater margins, is denied the nomination? Sure, they'll vote for Hillary in November (if they vote), but how much would that dampen their energy? To some extent, that probably would re-create '68. Not the police riot part or the obscenity-shouting mayor part, but the activists-angry-at-their-own-party part.
Late Update: I should credit Jonathan Bernstein for suggesting back in 2004 that this is the way the parties seem to be evolving. His Forum article on the rise and fall of Howard Dean suggested that networks of party insiders are increasingly using the primaries as beauty contests.