After trying various fixes, most of which were intended for Ubuntu, here's what worked for me:
- I booted the system off the Debian installer CD I originally used and selected "Rescue Mode" in the boot options.
- Proceeded through the options, selecting the defaults (network config, time zone, etc)
- Selected the option to open a shell at /dev/boot/root (your path might be different)
- Ran "lilo" to update the boot sectors on both /dev/sda and /dev/sdb (my RAID drives - LILO detected them automatically).