apt-get error: public key not available

Este fin de semana me paso el siguiente error tras añadir una linea en el /etc/apt/sources.list:

# apt-get update
[...]
Reading package lists... Done
W: GPG error: http://www.virtualbox.org etch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 390EC3FF927CCC73
W: You may want to run apt-get update to correct these problems

Tras buscar un poco por google, encontre como solucionar el error, importando la clave publica que faltaba, de la siguiente manera:

# gpg --keyserver hkp://subkeys.pgp.net --recv-keys 390EC3FF927CCC73
gpg: directory `/root/.gnupg' created
gpg: can't open `/gnupg/options.skel': No such file or directory
gpg: keyring `/root/.gnupg/secring.gpg' created
gpg: keyring `/root/.gnupg/pubring.gpg' created
gpg: requesting key 927CCC73 from hkp server subkeys.pgp.net
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key 927CCC73: public key "innotek GmbH (archive signing key) " imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
# gpg --export --armor 390EC3FF927CCC73 | apt-key add -
OK

Podemos verificar que la clave se ha importado con:

# apt-key list
/etc/apt/trusted.gpg
--------------------
[...]
pub   1024D/927CCC73 2007-06-04
uid                  innotek GmbH (archive signing key)
sub   2048g/F0C31E29 2007-06-04