at work i have 5 G5's. two are dual 2.7's and 3 are first generation dual 2's. with two of the dual 2's, dvd-rom and plain dvd's (movies) won't mount. on the other dual 2 it works fine and the 2 dual 2.7's work fine also. all of the computers have the same operating system (10.4.3) and all of the same updates. the two dual 2's that don't mount have worked fine in the past. i'm not sure when they started not working. it could have been with the upgrade to tiger (which was done with a clean install on all computers) or if it was something else. they all have quicktime 7.0.3. cd's and cd-rom's work fine on the two dual 2's that won't play dvds. permissions are repaired often. i'm at a loss as to why they don't recognize dvd media.
as far as media apps that might influence dvd usage, they all have after effects 6, the full final cut studio (fcp 5, motion 2, dsp 4, soundtrack pro, etc.) don't see how apps would effect the superdrive mounting media, just thought i'd add it.