Who else would even be in the primaries? Cuomo? He'll flee the minute he hears the name Clinton. O'Malley? His speech was possibly the biggest letdown of the DNC.
Clinton would clear the field for sure this time. Obama might even feel comfortable enough to back her in the primaries.
If it's Clinton vs...I don't know, the 2012 equivalent of Bill Bradley or something (i.e. someone with absolutely no chance whatsoever), then yes, Obama might support her during the primary season, since he wouldn't want to damage the eventual nominee.
I just have a hard time imagining that it'll be Clinton with clear field, though. She'll be 68, for one thing. For another, I have a hard time imagining that people like Cuomo, Mark Warner, Deval Patrick and Brian Schweitzer would just roll over for her. I think Obama will definitely support -- in some way -- the Dem nominee, but I think he'll wait until it's a sure thing.
Of course, whether his people wait is another matter entirely...