The developers of Canon hacked firmware "Magic Lantern" have begun porting the custom software to the 5D Mark III. Earlier today, Alex Dumitrache of the Magic Lantern firmware development group posted the image above of the 5D Mark III displaying a rudimentary "Hello, World!" string. This might seem insignificant, but it indicates that Canon hasn't taken drastic measures to prevent the firmware from being modified. The Magic Lantern Team has brought the custom firmware to the 5D Mark II, the T2i / 550D, and many other models.
The firmware is targeted at film makers, unlocking a variety of options and settings that Canon holds back right out of the box, like adjustable audio levels instead of an automatic gain control. However, photographers and videographers alike can enjoy zebra stripes for identifying blown out highlights or underexposed shadows, motion detection, enhanced exposure controls, extended bracketing, and several other features.
If you are a current owner of a Canon 5D Mark III and want to follow the progress of the project, be sure check out the Magic Lantern Wikia and the team's Twitter page. You can also take a look at some of the work done with Magic Lantern equipped cameras at their dedicated Vimeo page.