debian: Some packages could not be authenticated

On Debian 6, I've followed the instructions to get Debian 6.0 LTS working.

Now, when I try to upgrade, I get

WARNING: The following packages cannot be authenticated!
[...]
Install these packages without verification [y/N]?

Yet I have debian-archive-keyring:

# apt-get install debian-archive-keyring
[...]
debian-archive-keyring is already the newest version.

apt-get isn't really providing any useful diagnostics I can follow up. How would I go about debugging this error?