Problem:
When you try to update your package list with apt-get update , you receive several GPG error messages.

Example:
http://security.debian.org lenny/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA

GPG error: http://ftp.de.debian.org lenny Release: Die folgenden Signaturen konnten nicht überprüft werden, weil ihr öffentlicher Schlüssel nicht verfügbar ist: NO_PUBKEY AED4B06F473041FA


Reason:
after a while, the GPG key expires

Solution:
Update the debian keyring manually: apt-get install debian-archive-keyring
After that, you can run the apt-get update without GPG errors.

When you receive these messages regarding to debian-multimedia.org, you should do following:

Message:
W: GPG error: http://www.debian-multimedia.org squeeze Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907

Solution1:
download the debian-multimedia-keyring (I use wget):
wget http://www.debian-multimedia.org/pool/main/d/debian-multimedia-keyring/debian-multimedia-keyring_2010.12.26_all.deb

and install it with

dpkg -i debian-multimedia-keyring_2008.10.16_all.deb

after that perform a apt-get update

more information and maybe a newer deb-file: debian-multimedia.org

Solution2:
Install debian keyring package with apt-get install debian-keyring

and then:
gpg --keyring /usr/share/keyrings/debian-keyring.gpg -a --export 07DC563D1F41B907 |apt-key add -

That should do the trick :-)