I've had a similar problem, off and on for about four years. Tiger,
Leopard, now Snow Leopard. Disk utility (and command line
version) do not work. In the GUI, there is not an option for either
erase or quick erase when this happens.
The Mac sees the DVD-RW as empty, but would do nothing with it.
In four prior situations, the DVD-RW was "fixed" by using Windows
XP to erase/reformat it. Then it could be used with OSX.
With no XP systems nearby, I encountered another DVD-RW
that OSX could not deal with. I tried Tiger, Leopard, Snow Leopard,
on three different computers. I tried using a DVD/VCR to "initialize"
the DVD; it was happy, OSX was not. I tried using a consumer
VCR/DVD -- it was completely happy with "initializing" the disk for
recording but OSX was still not happy with it.
Versiontracker led me to a variety of programs. Most were just a GUI
for DiskUtility.
But using "simplyburns" the DVD became fully functional for reading, writing, and
re-writing with OSX.
http://www.versiontracker.com/dyn/moreinfo/macosx/28245