I wrote to a DVD-RW using my Superdrive-equipped iMac. I backed up my user folder onto it. I believe I did it with Finder.
[**Unverified tip:***] MacWorld (or MacAddict, I can't remember) had a tip about using DVD-RWs with iDVD. You have to first put in a DVD-R and then just before you click the burn button take the DVD-R out and replace it with the DVD-RW. I don't know if it works.