I had the same problem with Mac Pro 5,1 - dvd player crashes when dvd inserted. Did everything proposed by Apple Care, but narrowed down to hardware issue. Unfortunately the cost taking the tower to the service centre is more than replacement cost of new dvd player. The player itself was working reading data discs, it just crashed with movie dvd's.
Two things happened which go my player working - there was a minor update for ML, and I clean out caches using a cleaning app (thanks plopthegeek). It worked.
The issue I discovered could be due to the fact my Mac Pro came delivered with the wrong region setting. I'm in a Region 3 country, but it was set to Region 6. For this I frankly think Apple should have better Quality Control checks for a premium product like a Mac Pro.
Below is a screen shot of my Mac Pro dvd player with the wrong region code.