Mark Halperin, Time Magazine:

Actually overtaking Obama in enough states to win would require a combination of factors: Obama’s get-out-the-vote efforts have to turn out to be weaker than thought; young voters have to fail to channel their enthusiasm for Obama into actually voting; race has to be a bigger factor than most pollsters currently believe it to be; conservatives have to be more fired up than they have seemed; independents have to be more attracted to the Republican ticket than they have been all year; and, most of all, late-deciding voters have to break disproportionately to McCain.

Unlikely.  But not impossible.  Go vote.