I have a DP2 G5 and it wont mount DVD's I burnt on it a year ago. Mostly the problem is from Shintaro disks, so far. I think there has been a firmware update that has gone wrong. When did you get your mac and what is the model of your superdrive?
I burnt these disks in Toast mostly. Their settings were "DVD-Video" for the video folders and for data, I would burn the DVD-ROM (UDF) option half the time, and "Mac & PC" for the rest. I have about 50 Shintaro's that used to work fine and now don't. My issue is in mounting disks on my G5 that were burnt on my (same) G5 a year ago. I have not added any hardware upgrades and there is not much in the way of additional software on the system as I try to keep it pretty clean and lean. There are some software updates however. The OS updates to .8 & .9 in Panther were pretty poor for compatibility- I think this may be the issue but cannot be sure yet
I will be getting a cleaner disk this week to try and fix it. I have even reset my preferences (apple+option+P+R on boot) and played around with disk utility so there is not much more I can do. The only superdrive firmware update on apple is not for my drive and if the cleaner fails to work then I will have to get it to the Apple Store for a fix.