Standard caveats aside (it's early, we don't have a set field, blah blah blah), it's hard to see how Barack Obama loses the nomination barring scandal or the mother-of-all gaffes.
I've been working up a few scenarios given the primary calendar (which isn't set in stone, with states like California looking to move up), and really, it would be Obama's race to lose.
Iowa is right next door to Obama's Illinois, and while Vilsack will win it (getting no boost out of it), the race for second-place will determine the "true" winner. Hillary, for now, appears to be bypassing Iowa. So the early battle would appear to be between Edwards and Obama. A 2-3 finish for these guys, in any order, leaves them in good shape moving forward.
Nevada will be a battle between Edwards and his union allies, and Richardson and his southwestern and Latino base. New Hampshire will be fertile territory for Hillary and maybe Kerry (they hate Richardson because he pushed to insert Nevada into the calendar ahead of New Hampshire). And then South Carolina. With Iowa out of the big picture thanks to Vilsack, and New Hampshire diluted by Nevada, South Carolina may well decide our nominee in 2008. Richardson thinks he can win the state (I'm not sure how), and he, Edwards and Clark will stake their entire bids on the state. But given the state's large African American population, along with Obama's popularity with female voters (yeah, they love him), and it's tough to see how the rest, splitting the dwindling white male vote, can overcome those hurdles.
There's one thing that could put a skid on Obama's fast rise -- an Al Gore entrance into the race. Other than that, I don't see a way anyone stops him.
Again, we don't know what the final field will look like, so things can dramatically change. But an entrance into the race would make Obama the prohibitive favorite. If politics is about seizing opportunities, it would seem a no-brainer for him to enter the race now.
What's more, Obama would then be tough to beat in the general. He would very well be the favorite in that race, even against a McCain, and would probably be a net positive for Democrats running down the ballot. So it wouldn't be a terrible thing by any means.
(Tired disclaimer: None of this implies endorsement. I will say nice things and mean things about all these candidates before it's all said and done. As of now, I have no preferences or favorites.)