Btw, Dave... I just read this on canonrumors:
What about my EOS 5D Mark III?
It may not be as simple as just updating the firmware in the same way. If you remember the story about getting f/8 to work on the EOS-1D X with a Kenko teleconverter, Arthur Morris was unable to make the EOS 5D Mark III do the same thing with the exact same teleconverter. The fact that Canon hasnít updated both cameras at the same time tells me there are indeed differences in the AF systems. I am reminded only the EOS-1 line has ever had f/8 autofocus, so it may not be something that gets added to the 5D Mark III anyway.