I think McCain is going to do something crazy.
I have no doubt that it would be a bad idea -- I'm just not sure if that will prevent him from doing it. He's shown a willingness to act on plenty of other bad ideas.
He really seems to be telegraphing that he will go on the attack, and I think he hates looking weak more than anything else. (And he'd look weak if he says he'll do it and then doesn't.) So I think attacking will happen, but I'm not sure if it will be the regretful, statesmanlike attack ("It's a pity it's come to this") or crazy-person attacking (getting into Obama's personal space, interrupting, bringing up REALLY out-there stuff, etc.)