My user account is... missing. The files are still on disk, there's just no user with id=501.
So, tomorrow (well, today. Sunday.) I get to wipe the drive and reinstall again. I was able to make a couple copies of my home directory (single user mode for the win. or at least the lose-by-fewer points), so nothing critical is likely to be lost. And I don't think there was anything critical on it anyway. Everything in the workspaces was synced with Arcturus and/or Mithra, all the major user data was synced either with Dropbox or MobileMe, and the remainder was just copied of one of my other computers to begin with.
The annoying thing is that I had my backup script on the machine, and was going to poke at getting it running tomorrow.
This time, I'm going to take an image of the disk after I finish installing, upgrading, patching, and setting up my operating environment. Just in case something appalling happens again, I won't have to sit around for an OSX install, I can just re-image the disk.