Mailing the story:
When you install Debian without a mirror, you need to edit /etc/apt/sources.list if you want to use a mirror once the system is running
I'm surprised that I forgot how this works: When you install Debian without using a network mirror (either from the first CD of the full set or, my preferred method, with the DVD image), /etc/apt/sources.list is set up with a Debian mirror for security updates. However, there is no entry for the Debian software repositories, and if you want to add applications from a network mirror, you'll need to modify /etc/apt/sources.list accordingly.