My root filesystem filled up, and it caused all sorts of wierd effects. My n800 complained that some database was corrupt, and it was going to do some other ambigous thing (on startup). The sound volume was low, and couldn't be restored. Etc.

I tracked this down with xterm, du. It turned out that in the 'user' home directory, there's a '.thumbnails', and it had grown to 60 MB (presumably, it's all the free space i had at the time). I'm not sure which image viewer is doing this, though it's probably Quiver. I wish it would stop. Or at the very worst, it should put the thumbnails on the same card as the images. I have thousands of images. But, as a workaround, i copied the directory to an SD card with lots of space.

tar cf - .thumbnails | (cd /media/mmc2 ; tar xf -)
rm -r .thumbnails
ln -s /media/mmc2/.thumbnails

(my /media/mmc2 is an ext3 filesystem). Works for me.

After a couple reboots, everything's back to normal.