To the average voter, Bush and Kerry weren't that different. So, I'm not surprised that a significant percentage had a hard time making up their mind. But this time, the candidates are much different. Historically, that results in a landslide in favor of one or the other. But, it's not happening this time.
Much has been said about the "Bradley effect", which posits that some people are unwilling to admit to a pollster that they won't vote for Obama due to racial prejudice. That may account for some of it.
I'm wondering if some of the undecided voters are not willing to admit their choice to a pollster because of someone else in the room. There was a really popular thread yesterday about friendships and family relationships that are breaking up over this election. Maybe "undecided" is a face-saving position.
For that matter, maybe the choice of McCain or Obama is a face-saving position: tell the pollster one thing while someone else is within earshot, but cast a different vote in the polling booth. However, I don't know how you could quantify this effect.
I do think that most people have made up their minds, but will not state publicly...
“I’m wondering if some of the undecided voters are not willing to admit their choice to a pollster because of someone else in the room.” This is a fresh and maybe very workable explanation. Good job!