Beforehand, though, if Obama drops the cool veneer and layeth the smacketh down on McCain, it's all over. The key question will be if McCain brings up Ayers or ACORN, both of which are traps for McCain.
Otherwise, all Obama has to do is tell McCain directly, "Look directly into my eyes and tell me, and the world, that you have a plan that will rebuild America, create jobs, produce goods for export, fix our infrastructure, and get us out of the Middle East both economically and militarily." Then he just sits there and looks straight at him in a game of optical chicken. McCain will either give a bullshit answer, he'll turn away, or he'll lose it and go ballistic. In any case, Obama wins.
After that, the only possible chance would be for Sarah and/or Bristol Palin to pose nude in Playboy, but since Larry "Hustler" Flynt is proceeding with a Sarah Palin send-up porn flick, even that might not be enough.
UPDATE: McCain came out flailing and attacking. He fell into the Ayers and ACORN traps as expected, and in general looked very bad. Obama clearly won.
On the way out, McCain couldn't find the door, just like Bush did once, and wound up, once again, trailing Obama, to his great displeasure:
BTW, this photo was taken from a Yahoo News slideshow on the debate. If it's a photoshop job, blame Yahoo. If not, blame McCain. His campaign makes me react that way, too.