When you run apt-get in Debian Unstable, you can get a error like this:

W: GPG error: ftp://ftp.se.debian.org unstable Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 010908312D230C5F
W: You may want to run apt-get update to correct these problems

It can be solved, this way:
gpg –keyserver wwwkeys.eu.pgp.net –recv-keys 010908312D230C5F
gpg –armor –export 010908312D230C5F | sudo apt-key add –
apt-get update

Byt keyid “010908312D230C5F”, med den nøgle du mangler.