I have no idea what caused this, but I can give a pretty detailed account of what I did before the crash.

I have a world that I am making to place on a multiplayer server. I have been using the snapshots because I wanted some of the functionality that 1.5 will offer.

I had just fly-dug my way out of an abandoned mineshaft. I dropped gravel down the hole to cover it up, and put dirt on it to make it look natural. Then I pressed escape to pause it for 15-30 minutes. Then I came back, and I was flying around. I had just come back back my my chicken farm, (piston and redstone block make a 2-clock, which powers some droppers to get the eggs up to a dispenser used to throw eggs produced.) This farm has a large number of chickens in it.

This is when the game gave me a dull light blue screen and gave me the crash message. I have attached the log below.