I had problems mounting a DVD on my G4 PowerBook 867 -- the CD drive made funny sounds and just ejected the disc. However, Apple's Knowledge Base (article #25000) was very helpful, as it includes this tidbit:
Press the Command, Option, and I keys while inserting the DVD-Video disc. This causes the computer to read the disc as an ISO 9660 formatted disc, rather than a UDF formatted disc.I tried it, and was able to mount the DVD on my machine.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031103072051593