I agree that its unsafe, but I can tell you there are times when watching a DVD while moving isn't so bad.
Like waiting in a line of cars to pick up kids at the school... often times you can just sit in your car for about 20mins moving a foot or so at a time - so starting/stopping the movie would be a pain while inching up.
The DVD unlock code function is available on 04-05 XLRs. It was removed during the 06 model year build, so your vehicle may not support it.
1. With the engine running and the vehicle in PARK, press and hold the "note" key below the volume knob.
2. When the keyboard screen apppears, enter 1791, then press ENTER.
3. Remove the map DVD, and insert the movie DVD.
4. Shift into DRIVE and enjoy.
5. Be safe. We're sharing the road too.