hi,
Had something similar on my iMac G4 800. The DVD drive was making many attempts to read the brand new DVD and finally failed after few minutes (it finally ejected the disk). I rebooted and...it worked. This is the only DVD where I have such issue (i.e. worked ok with the iLife and iWork DVD's). I tried to mount it again after completing the Tiger install and had the same problem. Go figure...
Maybe reseting the PRAM ? maybe going to the store and changing disks ?
For info, my DVD player is a (old) DVD-104.
Hope you have this problem fixed soon.
Ciao