Political junkies have been going on at some length about how undecideds always break toward the challenger. The specific point is that if Bush and Kerry are tied going into the election, or even if Bush is in front by a bit, Kerry will win since the undecideds are certain to go overwhelmingly for him. In general, I think that's true, but in 2000 the undecideds broke toward Gore, not toward Bush, the challenger. Why? They suddenly had big doubts about his capacity to be president. Part of this was because of the DUI, but also because Gore was hammering Bush on the issue.
I don't think such an outcome is likely (though I wouldn't discount some sort of Rovian late-October surprise against Kerry), but it could happen. In short, don't count your undecideds before they break.