Thomas M. Holbrook reviews debates going back to 1988:
Across all 16 presidential debates the average absolute change in candidate support was just less than 1 percentage point. There are a few notable exceptions, of course. Two that stand out are the second debate in 1992, following which George H.W. Bush lost 2 points, and first debate of 2004, after which George W. bush lost 2.26 points. Other debates with above average (but still small) vote shifts are the first debate in 1996, the second debates in 1988 and 2000, and the first debate of 2008. Each of these debates has its own story, and I'm sure we can all think of anecdotes to explain the bumps and wiggles. Although the analysis is terribly outdated by now, my debate model from Do Campaigns Matter? came to the profound conclusion that the candidate viewed as having won the debate generally gets a small bump. I told you it was profound.
Silver finds "the first debate has normally helped the challenger":
In the nine elections between 1976 and 2008, there were only two years when the incumbent-party candidate gained ground relative to the challenger; these cases were 1976, when Gerald R. Ford halved his six-point deficit with Jimmy Carter, and 1988, when George H.W. Bush moved just slightly further ahead of Michael Dukakis.
But on average, the challenging-party candidate gained a net of one and a half percentage points on the incumbent-party candidate.
However, the challenger’s gains have come mainly from undecided voters rather than from the incumbent himself.
Silver goes on to guess that "Mr. Romney will gain a point or two in the polls by next week, while Mr. Obama’s number will hold steady."