I just found this. Probably you've already read it. It's few days old.
Canon Confirms “Light Leak” Issue in the 5D Mark III
I'm not too worried about shining a bright light into the back of my camera, with the lens cap on, while I'm shooting.
I'm also not going to be worried about how long my car will float in a lake, compared to other cars.
Neither am I going to be using a bright light, out in the dark, while I'm taking a night exposure. So that leaves, there is a real problem, please don't turn on the LCD during metering. (and don't shine bright lights in the back of the camera while you are shooting time exposures?)
Next question should be, does this affect other models? I can see it now, people bringing back their dark rooms, so they can put a lens cap on and shine a light in the back of the camera, in a perfectly dark room. Also I hope some Nikon people go do the same test. We could have an Earth shattering discovery here. (or a mountain out of a mole hill)
Let me put this another way. How many people in their lifetime, would have reason to be concerned about the LCD affecting the exposure 1/3rd of a stop in something near total darkness?