If you have the DVR-117 SuperDrive installed in your Mac, you can get faster read and write speeds and add dual-layer DVD support just by flashing the firmware of the drive.
The details of how to do this are here.
[kirkmc adds: The usual caveats apply: you could fry your DVD drive, cause a plague of locusts to infest your RAM, incite flooding in your toilets, and void your warranty. Use at your own risk.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060502082847891