Error message:
W: GPG error: http://security.debian.org stable/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A70DAF536070D3A1
This really is just your standard don't-have-the-gpg-keys error. So, get'em -- take the last eight digits from the long NO_PUBKEY string that is displayed on your computer. If you are using Debian 4.0, the above key is likely correct; if you are using Ubuntu or another version of Debian, it will be wrong. (The last eight digits are used as an identifier at the keyservers). Then:
gpg --keyserver subkeys.pgp.net --recv-keys 6070D3A1 gpg --export 6070D3A1 | apt-key add -
Repeat if necessary. All done, just do an apt-get update and no more warning!