You run apt-get update, and it complains about some missing key IDs:

W: There is no public key available for the following key IDs:
B5D0C804ADB11277
W: GPG error: http://volatile.debian.org etch/volatile Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY EC61E0B0BBE55AB3
W: You may want to run apt-get update to correct these problems

B5D0C804ADB11277 and EC61E0B0BBE55AB3 are the missing gpg pub keys. Fetch them with gpg, and import them into apt :

gpg --keyserver subkeys.pgp.net --recv-key B5D0C804ADB11277
gpg -a --export B5D0C804ADB11277 | apt-key add -
apt-get update

OR (new and improved!)

apt-key advanced --keyserver subkeys.pgp.net --recv-keys B5D0C804ADB11277
apt-get update

or, with a different server: apt-key advanced --keyserver pgp.mit.edu --recv-keys