I looked into it some more. It turned out that my .minecraft/saves folder was a symlink to a nonexistent directory, and on 1.5.1 (and 1.5 IIRC), this crash is fixed. The game still crashes if the saves directory exists, but Minecraft is missing read/write permissions for it, though. I will file a new issue.
Aaron Yanich
I looked into it some more. It turned out that my .minecraft/saves folder was a symlink to a nonexistent directory, and on 1.5.1 (and 1.5 IIRC), this crash is fixed.
The game still crashes if the saves directory exists, but Minecraft is missing read/write permissions for it, though. I will file a new issue.