I have not encountered many issues with my own 1DMkIII, but I may not be putting myself in a situation where the autofocus isn't working. I have had issues with walking lions and grass that impedes my view of the animal. On my older 1 series cameras I wouldn't have an issue with small things getting in the way, but now the 1DMkIII has super fast focusing, and no matter what settings I try I still have issues with the camera not locking on my subject at all. Back and forth, back and forth the autofocus goes.
I am typically shooting slow moving large mammals, which are not coming towards the camera. Now that I know what other people are complaining about, I will spend some time on the next safari trying to figure out how my specific camera behaves.