I’ve received the following info from Canon inc. Japan:
The movie color conversion matrix (RGB -> YUB) for the EOS-1D X and 5D mark III has been changed to BT.709 (sometimes referred to as REC.709) for Full HD and HD movies (On previous EOS products it was BT.601). SD movies remain as BT.601.
Therefore it is recommended that editors use the new version of the EOS MOVIE Plugin-E1 for Final Cut Pro (Ver. 1.3) that supports a BT.709 color conversion matrix when using Final Cut Pro 6.x/7.x for editing.
What does this actually mean?
BT.709 is a broadcast color space that uses RGB values of 16-235 (rather than 0-255 as per a still image). It is the standard colour space for HD footage.
The downside is that if not properly converted (i.e. converting 5D III footage to ProRes for editing) footage will be more susceptible to gamma issues with NLEs than the 5D mark II was. This will result in “crushed” blacks (dark areas without detail) and a darker gamma on a computer screen which uses a 0-255 color space. If transcoding with software other than the EOS-movie plugin (such as 5DToRGB) set the gamma to 1.22 to get the correct display of tones.
Basically so far Canon HDSLR’s did not conform to the BT.709 standard which is “recommended” for HD. While color spaces are mostly confusing and a cause of trouble for less broadcast experienced editors the fact that the 5D mark III now conforms to this standard makes it more easily acceptable for HD broadcasters.
Download the new EOS MOVIE Plugin-E1 V1.3 here: LINK
More info on REC.709/BT.709: Wikipedia