debian apt NO_PUBKEY error

A sample error message:

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

You can fix it by adding the missing public key by:

# gpg --keyserver subkeys.pgp.net --recv 54422A4B98AB5139
# gpg --export --armor 54422A4B98AB5139 | apt-key add -