As I type this, Stan Greenberg is briefing reporters on his focus group of undecided voters in Colorado. He said the respondents felt Obama "won" and that the results were "more decisive than either of the last two." That's a reference to Greenberg's previous focus groups, which also came away preferring Obama.
The most striking result came on the favorability ratings. Although the focus group was officially undecided, it leaned towards McCain. Here were the favorability-unfavorability ratings for each candidate at the start:
McCain: 54 favorable / 34 unfavorable
Obama: 42 favorable / 42 unfavorable
Here's what the ratings looked like after the debate:
McCain: 50 favorable / 48 unfavorable
Obama: 72 favorable / 22 unfavorable
Interestingly, Obama's most popular answer was about education. The focus group seemed particularly pleased about his riff on asking parents to take more responsibility for their children. According to Greenberg, this is likely part of the reason the focus group felt that, overall, Obama shares their values.
Update: Time's Amy Sullivan has a full write-up on Greenberg's results.