OK, taking the current figures from the Electoral Vote Prediction Site it looks to me like Kerry can win even if he is 2% behind in the national vote.
Summing across the data on the site and dividing by population, I find that it gives Kerry 46.4% of the national vote and Bush 47.8%, a difference of 1.4% in Bush’s favour, which is however not enough for him to win. The reason for this is that there are fifteen states where Bush’s lead is over 15% – Utah, Wyoming, Nebraska, Oklahoma, North Dakota, Idaho, Alabama, Alaska, Kansas, Texas, Tennessee, Kentucky, Indiana, Georgia and Montana – but only five where the same is true for Kerry – D.C. (technically not a state), Massachusetts, Rhode Island, Illinois and Maryland. 27% of Bush’s total vote is in those 15 states; indeed his total margin over Kerry in those states amounts to 12% of his total vote. If these polling figures are accurate (a big if), I reckon Kerry could be 2% behind and still win.