I shot an email to Canon, India and below is the response i got. I think it's fair.
As far as we know, this phenomenon does not affect the 5D Mark III in real world shooting conditions. This is a theoretical issue that was present in the 5D Mark II as well, it is just that no photographer shoots under such artich as shooting with a lens cap on, or shooting in total darkness with a long exposure. And with exposures longer than a minute, the 2 seconds of LCD illumination would not effect the exposure to any degree.
We feel that this has been highlighted with strong intent to undermine what is a strong product.
However, to calm the fears of persons who have purchased the 5D Mark III we are offering this service free of charge, and all models shipped after this problem was noticed do not have this problem.
I shoot with the lens cap on all the time!...LOL
This is really such a small issue. 2 years from now if your selling your mkiii the 'lightleak' non-issue will be totally forgotten and replaced with 2 years worth of thousands of photographers creating stunning images. Light leak?...don't remember that at all. what will lower the value is shutter actuations, has it been serviced, general wear and tear, how did those scratches get there, is there noticable dust on the sensor at f16-20+, , did you leave it in the car on a 100 degree sunny day....light leak, reallly, who cares....well, other than those who specialize in "shooting with a lens cap on, or shooting in total darkness with a long exposure. And with exposures longer than a minute, the 2 seconds of LCD illumination would not effect the exposure to any degree. "