This is electoral vote distribution, as predicted by the polling compiled by the site, over time. As you can see, McCain started out on top throughout the primary season, Obama pulled away after clinching the nomination, topped out with his Europe trip, and has fallen back into a tie now. This seems to fit the standard narrative pretty well. But the second graph tells a different – and I think more accurate – tale:
This is the same graph, only with "barelys" – leads of less than 5% – taken out of the equation. This only counts the states in which McCain or Obama have solid leads. Here, while there's certainly been some fluctuation, the overall trend has been a slow, steady rise for Obama and a jittery, zig-zag decline for McCain. In other words, the perceived "tightening" is only at the margins. Obama is still way ahead, and steadily pulling away. Obviously, at his highest level on this graph Obama doesn't top 270. He'll still need to squeak out some victories in toss-up states. But he has a lot more of a base to build upon than McCain does, a base which isn't deteriorating in the slightest.