Will undecided voters move toward McCain come election day? Nate Silver at FiveThirtyEight doesn’t think so -

The Undecideds

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A somewhat higher proportion of whites (and Latinos) are uncommitted, but the differences are not overwhelming.
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Another problem is that we haven’t been distinguishing undecided voters from third-party voters. There is an argument that third-party voters should be treated as quasi-undecided voters, since third party support tends to collapse at the voting booth. Nevertheless, Bob Barr and Ralph Nader will probably pick up a collective 1-2 percent of the electorate, and third party support tends to be overwhelmingly concentrated among white voters.
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Long story short … given optimistic assumptions (McCain wins 2/3 of white undecideds, 100 percent of third-party support collapses), the undecided vote is worth a net of about a point for McCain. Given what I’d consider to be more neutral assumptions, there’s no particular reason to think that the undecided vote favors him.

My guess is that the truth is somewhere in between and that this is worth, say, half a point for McCain. Even give him the full point if you like. This effect is probably smaller than that of the cellphone problem, from which there may be 1-2 points of cushion in Obama’s direction. If on top of that the polls are being overly conservative with their likely voter modeling, the numbers are more likely to be underestimating Obama’s standing than overestimating it.

It also should be noted that people still undecided at this point are more likely to simply skip voting altogether, further reducing any advantage for McCain. A sudden upset is looking less and less likely.