OK, so I managed to get my N800 booting from an external SD card, and it's been working fine for weeks. I took the SD card out of my N800 and popped it into a reader on my Linux box (fresh Jaunty install). For some reason, Jaunty wouldn't automount the card, so I just mounted it from the command line. I fiddled with some audio files and created an m3u file, then umounted it from the CLI and put it back into my N800.

Whatever I did screwed up my boot, as now the N800 just boots from the internal flash. It doesn't look like it even sees the SD card. HELP! What do I do? I've done an fsck on both the ext2 and vfat partitions, which repair a bad unmount I did on the ext2 partition (my linux newb is showing!), but that did not restore my external boot.