What Are The Key Battleground States

With all the talk of national polls, it's always important to remember that winning a presidential election is an electoral vote game.  I was looking at the electoral map on edwards site today and pondering which are the key battleground states.  I'd like to hear some thoughts about:

• Which blue states could be at risk?
• Which red states could be available for pickup?
• Where would resources be best invested to ensure a Democratic victory?