From TAPPED:

And so I offer three ways to interpret what's going on here: a) the Edwards campaign is irresponsibly punting on the question of being able to win a general election until it can get through the primary, despite stakes that couldn't be higher for the nation, and has private data that shows Clinton to be its major competitor (call that one the Markos theory); b) the Edwards campaign is making a short-term tactical mistake by ignoring the impending Obama threat while taking on Clinton; or c) Edwards is a person of principle who sees in the Obama campaign more of what he would like in the White House, and is going to go down in such a way as to try to take Clinton with him.

Or d) Edwards is holding out hope for an Edwards/Obama ticket.