Venturing slightly off-topic, but:
1a. I agree with Andrew Sullivan that Obama's demeanor was a problem. If you read a transcript of this debate -- you would probably call it a draw. If you were watching the TV with the sound off -- it would look like Obama was losing. Reality being somewhere in between those two things, the edge goes to Clinton.
1b. However, I sensed that Obama's mood was more one of exasperation than exhaustion. Obama had pivoted rather deftly from the bittergate controversy in recent days -- see for example his speech in front of the American Association of Manufacturers -- precisely because it reminded him of one of the original rationales for his campaign, which was running against the Washington establishment. e.g. "It's the career politicians in Washington who are out of touch, not me", or some variant thereof. However, it was impossible to strike that tone given the nature of the questions, which were more designed for superdelegates than ordinary voters. Obama faced a frontrunner's scrutiny, even though he's behind in Pennsylvania, an inherently difficult position for him not made better by the moderation. But Obama can be faulted, I think, for not gaming out a tonal strategy for this type of debate.
2. With that said, debates are won and lost in the 24-96 hour time period, rather than on the evening of. There weren't really any YouTube moments in this debate, and while the media is likely to focus on things like the Ayers issue --this is the same media that has consistently misread the pulse of the American public over the course of this campaign. The media badly misread where bittergate registered on the Richter Scale; they also badly underestimated how the "pile-on" narrative -- and their own slobbering praise for Obama -- would play out for Hillary Clinton before the New Hampshire primary. There are elements of the media -- see a good example here -- that are still focused on the 1998 model of winning elections. But this is a 2008 universe, and the public is both more battle-weary and far more sophisticated in the way that they consume information. This is arguably the same problem that the Clinton campaign has had for much of the primaries.
3. In terms of Pennsylvania, I think this can most safely be regarded as a missed opportunity for Obama. One thing we haven't mentioned is how few undecideds there are in Pennsylvania -- as few as 5-7 percent in many polls -- and those that have selected candidates are pretty dug in. I would assume that an undecided voter who had doubts about Obama would not have those doubts erased by tonight's performance -- but there are also not a lot of undecideds. It's become a very stubborn electorate.