Canon hasn't used the word Light leak. I am sharing the product advisory for 5D III camera. Just wait for sometime and you will hear the actual story. Till then rumour mongers will rule. As far as 400 f2.8 vs 500 f4 lens, we will discuss in a separate thread.
Sabyasachi

Product Advisories
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.

Phenomenon
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.

Affected Product
Canon EOS 5D Mark III Digital SLR Camera

Support
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.

Thank you,
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
TDD: 1-866-251-3752
Email: carecenter@cits.canon.com
For additional support options: Canon U.S.A. : Service & Support
Source: Canon U.S.A. : Support & Drivers : Consumer & Home Office