Two theories about what's going on on the Dem side: 1.) It really is going to be a nail-biter--the polls over-sampled independents, who ended voting in the GOP race (for McCain) at a much higher than expected rate. (The Prospect's Tom Schaller, standing right next to me, suggests people assumed Obama had it wrapped up and wanted to put McCain over the top.) 2.) Obama will win--several precincts ran out of Dem ballots today because of a surge of independents who wanted to vote for Obama. You'd expect those precincts to come in late--since they had to wait around for ballots.
P.S. Just getting some exit poll data from MSNBC that suggests theory 1) has something to it. Hillary's cleaning up among women and late deciders--people who decided in the last three days.