I had the hardest time getting my GTS 450 to work. The last straw was the PCI root fix in Multibeast. After that OSX finally connected with the video card. It still says "unidentifed Nvidia card" in the profiler, however everything is fully functional. I even have FCP 7 running on this beast like a champ. Perhaps trying the PCI root fix would save some people a bit of time in the future.
--------------
EDIT: My GTS 450 stopped running FCP playback, also QC/IE is not enabled. I have run into the "fermi freeze" a few times as well. Running VLC in the background has apparently fixed the "Freeze" for now... I made this post almost immediately after configuring my system, and after much experimentation have found this video card to be a nightmare and a half.
Runs Starcraft2 like a champ though, so it's a good gaming card but has some problems w/ FCP7.