The 5D MkII has the same af system as the 5D ('classic'), which at the time of the 5D's introduction was Canon's 2nd best af after the 1D series (and EOS 3). It was designed to cover a greater horizontal area than the 20D's 9 point af system, as well as having 6 assist points when using AI Servo mode. Thus it was designed to be superior to the XXD series without treading on the 1D series' toes. I don't buy the argument that the 5D series was never designed for action. It was obviously not designed to be as capable as a 1D series, but not all action is professional sports photography, even 5D owners want to be able to take pictures of their kids running around!
The 5D series af system fell behind the XXD series when the 40D was introduced and for whatever reason, Canon did not choose to develop a new af system for the 5D MkII. Possibly this was because Canon believed that the 21MP sensor and HD video was enough of a selling point that they didn't need to go to the expense of developing a new af system; possibly the D3 generation Nikons caught them with their pants down (i.e. too late in the 5D MkII's development programme to respond to the fact that Nikon were going aggressive on af).
With the launch of the 7D, the 5D series has fallen even further behind its juniors and I don't think that Canon will be able to get away with anything less than the 7D's af system this time around (certainly not with 'only' 22MP). To me, it makes no sense to develop a brand new 9 point af system for the 5D MkIII (or whatever they plan to call it). Why go to all the development expense for a system that is inferior to a lower model in the lineup? I can't see that even the 1D X's 61 point system is actually that much more expensive to make than any other in Canon's lineup once the development costs have been met. OK, for really budget cameras, where every penny counts (like the Rebels) this matters, but for the higher price cameras, I don't think that the af system is actually a major cost consideration, otherwise how could Nikon put their 51 point system in the D300? I believe that it is mostly for marketing reasons that all Canons below the 1D series have inferior af systems.
I don't think I disagree with most of that.
I think there are some significant differences between the 51 point system in the D300 and the 61 point system in the 1D X, though. For one, the D300 AF system only has 11 cross-type points, its frame spread is less extensive, and it only has cross-type sensors in the center. The 1D X AF system has 41 cross type points, in three groups...center and to both sides. All of its sensors are double strip sensors....both single line, cross type and high precision (diagonal) cross type. The 1D X AF unit also apparently has the most extensive frame spread of any AF unit to date...from what I've read and the videos I've watched, its pretty much at the physical limit...any greater spread would require some radical rethinking of how an AF unit works due to light falloff as you approach the edge of the lens (a real problem, since the AF unit effectively polarizes light and splits the remainder into two beams that are then used to sense phase shift...not a lot of light to work with period.) The 1D X AF unit is far more complex and much more advanced than the D300 AF unit. Its also still buried under R&D costs that would need to be recouped.
So, I do think there could be a cost, possibly a hefty cost, to using the exact same 1D X AF system in the 5D III/X. Its not quite the same class as the D300 AF unit...its a class beyond.