When I join single player in 1.6, it immediatly crashes, in both survival AND creative.

I went to go play the new Minecraft 1.6.1 in the new launcher. I wanted to test it out in single player first. I created the world, and it immediately crashed. It's never done that before, the only time I've EVER crashed is when setting off over 5000 TNT. Please fix this bug, because I can't play Minecraft. I get this bug on both survival AND creative.