W: GPG error apt-get update NO_PUBKEY AED4B06F473041FA

error message:

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

reason:
as it turns out this is because GPG key expired

This key rollover is a normal maintenance task and was started in August 2010. For security reasons Debian’s archive signing keys regularly expire after three years.

solution:
update the keys by

sudo aptitude install debian-archive-keyring