I wondered for a long time how to erase a CD-RW with OS X. In fact, this is very simple (though I don't really like this method): it is done with Disk Utility (in Applications/Utilities).
I don't like it, because with this program, you can also erase any disk! I would have liked to create an Applescript to do it, but Disk Utility isn't scriptable...
[Editor's note: Although basic, this is one of the major changes with OS X. You no longer can erase a disk from the Finder - you must use Disk Utility. Keep this in mind when you look for "Erase..." in the Finder only to discover it's not there!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011105104409691