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

#apt-get update
.......
........
W: GPG error: http://www.debian-multimedia.org etch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907
W: You may want to run apt-get update to correct these problems

To solve this problem

# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys 1F41B907
# gpg --armor --export 1F41B907 | apt-key add -

then again execute "apt-get update".