MacInTouch had a report today about a workaround for Apple's crippled superdrive firmware which doesn't allow DVD-RW. Normally you would hop over to the Superdrive firmware page of the incomparable Cynikal and download a firmware update to enable the RW features of your drive, however if you're worried about ruining your drive, or your drive is too new for a firmware update, then the MacInTouch solution may be more appropriate.
Edit: due to MacInTouch's terrible layout and constantly changing URLs (the above link from yesterday is already broken!) I'll repost the relevant passage here:
If you put a fresh DVD-RW in your Superdrive, it spits it back out. However, get a friend with a DVD-RW on a PC to write something onto it. This will mount on your Mac just fine. And guess what? Toast will now erase it and from now on it can be used like any DVD-RW! It's as killer workaround that I have tested with no problems.