Don't know - I was at that Letterman show and the scarf looked odd to me too. The Letterman theater has the reputation as being super chilly (maybe because David Letterman likes it that way?) but that night the theater was actually quite warm. Perhaps Jon was just being prepared for chilly. I have also heard that he was sick that night. But the next night was the Best Buy Theater show and he showed no sign of being sick at all. His voice certainly wasn't off.