This was a relatively dry debate. Both candidates basically held their own, no one made any major gaffes, and there weren't too many big attacks.
I can see arguments why such a debate could help either candidate. Independents are leaning Obama but want to know that he has the maturity and thoughtfulness to be president; he showed tonight that he does. On the other hand, McCain had a really crappy week in which he looked pretty unstable; tonight he looked stable.
My guess is that this debate won't affect the state of the race very much. As Tom Holbrook reminds us, most debate effects are pretty negligible, with the exception of 1992 (after the Richmond debate in which Clinton connected with the audience and Bush kept checking his watch) and 2004 (after the first debate when Kerry skewered W on Iraq). There were no such moments tonight.