I'm with Steve. I doubt that this debate changed many minds. Kerry marginally outpointed Bush, and the president was genuinely annoying during the first half. But he recovered well, and actually presented himself pretty well during the discussions of domestic policy. Kerry's favorable ratings will probably rise a tad (after all, many Americans still know little about him), and he might gain a point in the polls.
Viewership was probably low for this debate, given that it was a Friday night on a holiday weekend, and that it was up against the baseball playoffs, and since there was no obvious moment to be replayed on newscasts (and who watches the news on Saturday anyway?), the Showdown in St. Louis will probably fall into the weekend news hole, never to be seen again. By Monday, all eyes will be on the Dustup in the Desert. (All bets are off if SNL manages to show Bush physically assaulting Charlie Gibson or the man who asked him about importing drugs from Canada).