Toward the end of the installation, the Debian installer told me that it saw I had Ubuntu installed and so would I want to install GRUB? It also assured me that it recognized that I had Ubuntu installed and that everything would be ok.
I've been through similar prompts with dual-boot installations before, and indeed everything has always gone just swimmingly. So, I said, "Sure." What I didn't know was that Squeeze is using the new and possibly-not-entirely-ready-for-prime-time GRUB 2. (I guess Karmic is as well.)
When I rebooted, GRUB gave me options for the new Squeeze install, but any trace of the Ubuntu install was gone. Hunting for /boot/grub/menu.lst gave me nothing. That's when I discovered that Squeeze installed GRUB 2 and most of the GRUB repair stuff on the Internet was obsolete. I was about the resort to drastic measures when this post inspired me to try
and it worked.