From the forthcoming David Plouffe book:
Sometime after the South Carolina debate Plouffe got a call from a senior Edwards Advisor who said Edwards was willing to announce the end of his campaign and join forces with Obama to defeat Clinton. When Plouffe asked if he could raise this with Obama the Edwards advisor said, “Yes.…Just to be clear we’re going to talk to the Clinton people too. That’s not where John’s heart is, but he is at the point of maximum leverage now.”
“Obama’s answer,” Plouffe writes, “was quick and firm: he would cut no deals.”
Interesting stuff, I definitely have Plouffe's book in my amazon.com queue.