When doing apt-get update you might see a lot of errors like
W: GPG error: http://ceph.newdream.net lenny Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DA4420ED288995C8
W: GPG error: http://download.opensuse.org Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 85753AA5EEFEFDE9
W: GPG error: http://ppa.launchpad.net karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 28577FE31F882273
W: GPG error: http://download.virtualbox.org lenny Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 54422A4B98AB5139
For the best part you should install the apropriate keyrings
apt-cache search keyring$
should list most of them, sometimes they dont exist for some third party repositories, so try this one liner, split for (a little) clarity
for KEY in `apt-get update 2>&1 |grep NO_PUBKEY|awk '{print $NF}'`; do gpg --keyserver subkeys.pgp.net --recv $KEY; gpg --export --armor $KEY|apt-key add -; done
Caveat, this is insecure, but more secure than disabling validation. Please be aware for full security you should validate the key signatures you are importing via private quantumly secured links to the originator obtained at your own cost, etc etc.