The planned firmware update will add two major tweaks: uncompressed HDMI pass-through for video, and cross-type focusing up to f/8. This is the same AF tweak that Canon recently released for the 1D X. Here's the official description of the changes:
Uncompressed HDMI Output Support
When shooting video, HDMI Output makes possible the recording of high-definition uncompressed video data (YCbCr 4:2:2, 8 bit) from the EOS 5D Mark III to an external recorder via the camera's HDMI terminal. This, in turn, facilitates the editing of video data with minimal image degradation for greater on-site workflow efficiency during motion picture and video productions. Additionally, video being captured can be displayed on an external monitor, enabling real-time, on-site monitoring of high-definition video during shooting.
Improved AF Functionality
Even when the EOS 5D Mark III is equipped with an extender and lens making possible a maximum aperture of f/8, the firmware update supports AF employing the camera's central cross-type points (currently compatible with maximum apertures up to f/5.6). Accordingly, the update will allow users to take advantage of AF when shooting distant subjects, benefitting sports and nature photographers, particularly when using telephoto lenses.