When upgraded form Sarge to Etch I started to notice a warning when using apt-get install from command line. The warning said:
WARNING: The following packages cannot be authenticated!
Googling with the warning text as a keyword turned out (Debian style!) simple solution:
apt-get install debian-archive-keyring
apt-get update
Nice to know that this is becouse Debian offers another level of safety in installing packages.
Little bit more about this topic is avilable at the origin of the above solution:
How to solve “The following packages cannot be authenticated” – The Changelog