If Gallup and the other polls show some closing in the margin that may not be to surprising. I've cited the 1976 race previously where Ford closed a similar mid Oct gap to get within 2% by election day. Gore likewise was the candidate of the incumbent party and closed a lesser gap to take the popular vote.
The people who haven't already decided to vote against the incumbent party by the time early voting starts are overwhelmingly likely to never decide to do so. As much hay has been made of this election being a referendum on Obama (i.e., the dominant media narrative), it is really about Bush. More specifically, it is about the degree to which Independents ascribe to McCain the specific things about Bush that they dislike. If someone has yet decided that McCain is too much like Bush to support by now, they likely will not have done so by the time that they vote.
