Recent aptitude update replied with
...
W: There are no public key available for the following key IDs:
A70DAF536070D3A1
W: You may want to run apt-get update to correct these problems
Checking the recent archives for the debian-user mailing list, I found this thread, with the easiest answer being
# gpg --recv-key A70DAF536070D3A1
# gpg --export A70DAF536070D3A1 | apt-key add -
I'm guessing that works because the key is in the key server (subkeys.pgp.net for me), but it hasn't been added to the debian-archive-keyring package yet. This email led me to that conclusion.
Update 2007-07-04: The recv-key line may include the keyserver, like so:
# gpg --keyserver keyring.debian.org --recv-key A70DAF536070D3A1