ISO filesystems are helpful for creating disk images for burning CDs.
On Nix:
mkisofs /tmp/isoname.iso -R -V "Volume Name" /path/to/iso/dir
If you have a DMG file you would like to convert to an ISO, you might find this article helpful on macosxhints.com:
http://www.macosxhints.com/article.php?story=20040121135301830&query=dmg+to+iso
