Many people have been reporting an issue with the 5D Mark III that Canon has now finally addressed. It seems that when the top LCD backlight is turned on in a dark environment, it's possible that the camera's automatic exposure will be affected and the resulting photos will not look correct. For video, this is a non-issue, but the fact that Canon is possibly going to fix or replace 5D bodies out there is a pretty big deal. They won't go so far as to call it a recall, but it seems that essentially all cameras that have been released up to this point may suffer from the issue.
Here is the full response from Canon about the issue:
To Users of the Canon EOS 5D Mark III Digital SLR Camera
Thank you for using Canon products.
The phenomenon described below has been confirmed when using the Canon EOS 5D Mark III Digital SLR Camera.
Canon is now examining the countermeasures and once the countermeasures are decided, we will post the information on our Web site.
In extremely dark environments, if the LCD panel illuminates, the displayed exposure value may change as a result of the AE sensor’s detection of light from the LCD panel.
Canon EOS 5D Mark III Digital SLR Camera
Once the preparations are complete, we will be making an announcement on our Web site.
This information is for residents of the United States and Puerto Rico only. If you do not reside in the USA or Puerto Rico, please contact the Canon Customer Support Center in your region.
Please register the EOS 5D Mark III. By registering, we will be able to notify you via email when service updates are available. If you already registered, please ensure you are opted-in to receive the notification.
Customer Support Operations
Canon U.S.A., Inc
Contact Information for Inquiries
Canon Customer Support Center
Phone: 1-800-OK-CANON 1-800-652-2666
For additional support options: www.usa.canon.com/support
So is it possible that this could affect your stills shooting at night? Maybe, it really depends on whether you are more comfortable changing settings while looking through the viewfinder or you would rather look at the top LCD with the light on. If the light is off, there are no issues. If you forget to turn it off yourself, it will turn off after a certain amount of time. Keep in mind this is a metering issue - it does not actually affect the exposure directly. It is good that Canon is officially recognizing this problem instead of ignoring it, and it seems like they are going to come up with a solution in a timely manner, as those who shoot at night with automatic exposure could be affected by this in a great way.
For most people who've tested this, it seems many cameras can be affected by having the top LCD panel light turned on while shooting photos. The exposure can also be affected by not shooting with your eye pressed against the viewfinder because light can bounce around inside - something all motion picture film shooters know something about. But the good news for video shooters is that it's not an issue in live-view mode since the mirror flips up and there's no light leaking to the sensor. I think talking about issues like this is important, because for those with 5D Mark IIIs, they might actually be affected by this problem. But it's always best to see what the response is from the company before overreacting, and while it does seem that there is a problem, it doesn't seem like a serious one - and Canon will be working out a solution.
So the good news is that if you're a Mark III owner and you only shoot video, you're not going to be affected in any way.