There were several times throughout the debate that I wondered if Romney wasn't on speed. The grin, the incessant blinking - the fact that he was almost doing some kind of weird monologue that was only partially grounded in sanity. There were a few times I watched the debate without sound, and Romney's body language kept saying "I'm crawling out of my skin!"
I really get a feeling about 30% of the way through, Obama realized that he was debating someone who would say anything, even the most nonsensical, and that if he tried to refute it he'd just get caught up in a no win argument. Like coming to play a chess match and finding your opponent is playing Super Mario Brothers instead.