I'm running a 3.6GHz i5 iMac with OS 10.6.5 and an ATI Radeon HD 5670 (512MB) and have been experiencing the same problem that flashadams described above - The standard mouse cursor being perpetually stuck in the middle of the screen while I play.
Curiously, the cursor doesn't show up in screenshots taken using CMND + Shift + 3.
Unfortunately, I don't have a framework to delete to fix the problem. From what you guys have said I guess I'll be waiting on Apple, which is to say I might be able to play Amnesia over Summer break. Ah well. It'll be just as good then as now.
EDIT: Success! As of the OS 10.6.7 update I'm no longer experiencing this issue on my iMac and can play the game as intended.