Just to put this all into perspective, I have one with a serial "1" and have not experienced ANY issues with the camera at all, even with firmware 1.0.7. I upgraded the firmware anyway maybe to prevent something from happening during a critical shoot.
I have not specifically checked for the light leak issue but I have shot in both bright sun, shade, combinations. I've taken low light shots, shots of star clusters, and shots by candle light. The reality is that I dont see the issue, or any of the reported issues, on my 5DIII.
So for those having issues, you should return your camera if you are not happy. For those thinking about buying one, you should just do it and buy from a reputable dealer with a good return policy so you can evaluate it yourself. I dont see the camera as "defective", not yet anyway and while all of this bickering is going on, I will be out there taking better pictures than those with a lesser model.