You might also try a different brand of DVD-R. a few years ago, brands made a difference, and some brands just never really worked out on certain DVD Burners. that problem should have been resolved by now, but you never know.
I have the same problem with my mbp. Memorex DVD-R's just wouldn't get mounted. When I tried another brand, it mounted but iDVD wouldn't burn to it, yet I could still burn normal files to it.
i JUST started having this problem a few days ago. (iMac 20" G5)
it's driving me insane because it will accept a blank DVD but after it's burned it won't accept the DVD anymore.