I had a similar hunch, but about 3-4 percentage points. Here is how I arrived at it. Assuming that the opinion polls are accurate, the split is about 50/50 - meaning nearly zero percentage point difference between the candidates. I assume, furthermore, that opinion polls are unlikely to capture newly energized and registered voters, and that these are largely against Bush by the factor 2:1. It is so because those who did not vote in the past and register now are more likely to those who are pissed off at the incumbent - happy campers tend to stay home. That gives the 67/33% split or about 33% lead for Kerry among newly registered voters. I furthermore assume that there will be about 5-7% upsurge of voters comparing to the usual voter turnout of about 50%. That translates into about 3-4 % lead for Kerry in the total popular vote.
Wojtek