Isaac, can I divert you from any such paths?  I implemented this once
using CD's.  The problem with optical media and tar is that tar is a
sequential backup utility, whereas optical media is best used as a
random seek device.  In my experience, in order to extract a single file
from a multi-volume backup (spanning multiple CD's or DVD's), you MUST
read in the entire volume.  (There may be ways to shortcut this, but I
never did find a way when I was using this paradigm about 5 years ago.)
If you have 3 DVD's with 4.7 GB of data on each, and your file exists on
the last DVD, tar will scan SEQUENTIALLY the first two DVD's before
finally getting to the third to restore your one file.

If you are not planning on restoring individual files (who ever plans
for such things?) and care only for an archive copy, this might be an
acceptable method.  Personally, I would rather write compressed versions
of these files to the DVD individually.

There is a new option (new, as in I didn't know about it) for growisofs,
-z, which generates RRIP records for transparently compressed files.
You use the mkzftree utility before running genisoimage.  This only
works for Linux 2.4.14 or later and creates a non-standard Rock Ridge
extension that can only be read on-the-fly by Linux servers.  To
decompress files on other OS's you need to use mkzftree tool by hand.

There may be backup solutions designed with this in mind for CDR and DVD
backups.  I haven't looked for them, so I don't really know what's out
there.  Personally, I would much rather buy a couple external SATA disks
of sufficient size to do system and data backups, then move one
off-site.  You can use dm-crypt interface to encrypt entire partitions,
then use simple tar routines to back up files (or rsync).  Stick on in a
safe deposit box, and exchange it once each month with the one on-site.

I would LOVE to see data deduplication make its way into the Linux
device manager as a block-level module.  Free deduplication for any file
system... mmmmm...  I wonder if anyone is working on that.  Perhaps it
would be a good PhD thesis...

Chad