People will get excited about a 12fps camera like I got excited when I got my 1D IV. But the only way my camera can do 10fps is in one-shot focusing. Put the camera in AI servo and there is no way the camera will focus quickly enough to maintain 10fps... Specially if the subject is running towards you... Even with 10 processors, the AF system needs time to accurately focus. You can tell the camera to shoot anyway, but who cares for an out of focus image anyway...
I hope the new AF system on the 1D X will be able to "keep up" with the camera's fps...
Your observation got me interested in what kind of qualifications Canon put on its "12 frames per second" rating (something that I am looking forward to), so I looked it up in their new online manual, and here is what they say (assuming you can decipher it):
From p. 111 of the manual, "The maximum continuous shooting speed of approx. 12 shots/sec. is attained under the following conditions*: At ISO 25600 or lower, 1/1000 sec. or faster shutter speed, and at the maximum aperture (varies dependin on the lens). The continuous shooting speed may be slower depending on the ISO speed, shutter speed, aperture, subject conditions, brightness, lens type, flash use, etc. *With the AF mode set to One-shot AF and the Image Stabilizer turned off when using the following lenses: EF 300mm f/4L IS USM, EF28-135mm f3.5-5.6 IS USM, EF 75-300mm f/2-5.6 IS USM, EF 100-400mm f/4.5-5.6L IS USM."
More disclaimers than the end of a used car dealership commercial! Can anyone say whether the asterisked text at the end is intended to mean that you only get 12 fps in one shot AF mode AND with those lenses, or whether instead they are saying that those lenses are a special case, and for them you have to use one-shot AF and turn off image stabilization?
Something got confusing in the translation. Or maybe it's intentionally confusing?