Here's another thought I had: neither candidate has been spending much time in Colorado, and it's a state Obama HAS to have if he can't flip Florida, Ohio or Virginia. If Obama loses the latter three, Colorado is a must-win for him.
So, with that in mind, why isn't Obama making more of a play there? Why isn't McCain maintaining more of a defense there? Why is McCain spending so much time in Pennsylvania? Is the race closer there according to internal polling, or is McCain making a strategic decision to contest a Kerry state because he knows Colorado, along with Iowa and New Mexico are lost? Remember, Obama wins this thing if he's got the Kerry States + Iowa/New Mexico/Colorado. He wouldn't need Florida, Virginia or Ohio.
Iowa and New Mexico are definitely going blue, so Colorado is a potential clincher. If Obama has Colorado, McCain has no choice but to try and flip a blue state.