This thread shows what's happening with the front of some:
http://www.camaro6.com/forums/showthread.php?t=438521
When I was discussing this problem with my dealer, they pointed out a similar issue with the front of mine. Mine isn't sagging in the front though, so it's fairly hidden by the tinted border around the windshield. You can see it if you're really looking for it. Honestly, that didn't bother me too much.
The edge of the headliner showing so obviously to any passenger who sits in my car is concerning though. As mentioned above, it's embarrassing.