I do have a question for everybody saying Canon should just put the 7D's AF system on a full fram sensor. Surely the AF system would only cover a very small portion (center only) of the bigger sensor? So essentially a 19 point AF system for a FF sensor would be a completely new AF system?
Probably. But actually, not as much smaller as you might think! If they compensate for the larger frame, it means developing a new sensor. If they don't, it means the 6D would have more AF points, and better ones, but the area coverage would be a bit less than that of the 5DII, as you can see in the attached image (5DII in blue, 7D in black, not adjusted for the relative sizes of the image sensors).
The AF area coverage of the 5DII was already not a strong point. When they released the 5DII, they touted the fact that the horizontal point spread was the same as the 1DsIII, conveniently ignoring the fact that the vertical spread was a full row shorter. The 7D's AF sensor on the 6D would mean the loss of a horizontal row, as well.
But even though those outer points would not be very far from the center, they'd be better than the 5DII's AF outer points...