Let me first say that the much discusses light-leaking issue is nothing that would seriously affect your images. And there is no reason to return your camera or to cancel pre-orders. However, a new 5D Mark III owner wrote to Canon and got this answer:
Further to your enquiry we would like to inform you that we very recently (in April) have become aware of this and is now a known issue with the EOS 5D Mark III model. The AE sensor in the camera detects the light from the LCD panel when it is turned on and the exposure value will be altered. The change is not significant as it will be altered by approximately 1/3rd of a stop but can be noticeable. You can continue to use your 5D Mark III and the LCD screen can be turned off to receive the correct exposure.
Check the videos to learn more…
[via dpreview forum]