In case you did not pick up on this fact from the Canon-published 1D X AF information, the new Canon EOS-1D X, unlike previous 1-Series DSLRs, will not autofocus when using lenses or lens combinations with an f/8 max aperture.
Chuck Westfall, through Arthur Morris, confirms this:
"AF is unavailable on the EOS-1D X if the maximum aperture reported to the camera through the electronic lens mount is smaller than f/5.6. This is a lower specification than previous EOS-1 series DSLRs. On the plus side, consider the fact that with most f/4 lenses including the 400 DO, 500/4L IS and IS II, and 600/4L IS and IS II, you now have 41 cross-type AF points plus color and face detection, whereas you had no cross-type points and no color or face detection during AF with previous EOS-1 series DSLRs using the same lenses, not to mention a significantly wider AF coverage area from left to right."
This is of course disappointing to many extender users. Canon does not currently have a bare lens with an f/8 max aperture, but adding a 1.4x extender to an f/5.6 max aperture lens results in an f/8 max aperture lens. And, adding a 2x extender to an f/4 max aperture lens results in an f/8 max aperture lens.
The largest group of photographers affected by this change are, probably, bird photographers (such as Arthur Morris) - who have been dealt a double blow with the 1D X. Bird photography typically needs the most reach possible - through both lens focal length and sensor density. Increasing focal length via extenders has now been limited (for autofocus to function) and the 1D X has a less-dense sensor than either of the previous 1-Series models.
There are still some Canon EOS-1D Mark IV bodies available.