Canon has issued a firmware update for the 5D Mark III.  Version 1.1.3 adds the following:

1. Supports the single focal length EF40mm f/2.8 STM lens that is to be released at the end of June, 2012.
2. Fixes a phenomenon in which an image may become underexposed when using the Auto Lighting Optimizer for continuous Auto Exposure Bracketing (AEB) shooting.
3. Fixes a phenomenon in which the backlight of the LCD monitor may not turn off depending on the camera settings and timing.
4. Fixes a phenomenon in which the camera's power may not turn on when a super-telephoto lens*1) is mounted to the camera with an extender*2).
*1) EF 300mm F2.8L IS II USM / EF 400mm F2.8L IS II USM
*2) Extender EF 1.4X III / Extender EF 2X III
5. Corrects mistakes in the Dutch, Italian, Korean, Norwegian, Polish, and Swedish menu screens.

You can download the update from Canon's website.