A reasonable way to achieve a long term backup of OpenPGP (PGP, GnuPG,
etc) keys is to print them out on paper. Paper and ink have amazingly
long retention qualities - far longer than the magnetic or optical
means that are generally used to back up computer data. A paper
backup isn't a replacement for the usual machine readable (tape, CD-R,
DVD-R, etc) backups, but rather as an if-all-else-fails method of
restoring a key.