Obama has been dodging a real debate with McCain for months.
McCain saw himself between a rock and a hard place, just like the RNC convention during the hurricane.
He could:
A. Go to DC and do the job he's currently paid for (and act as he has in the past, to help broker a compromise).
B. Act as if it's not his problem, and campaign like nothing else is going on.
He picked Option A. Obama wanted B. I don't think I'd blame either one of them, really.
I find it interesting, though, that Obama didn't use this as an opportunity to prove he could do what he claims: reach across the aisle. Personally, I think he doesn't want anyone to see clearly that he doesn't have that skill -- if he had it, he might have used it earlier. But if he actually CAN do as he claims, he was an idiot not to take an excellent chance to demonstrate it. That might have won him the election.