I agree - not sure I really understand your point though - in a nutshell, any camera is free to go as fast as it's able until the buffer fills - and from there on in it's frame rate will be directly proportional to the speed at which the camera can write from the buffer to the card (assuming zero processing latency which is pretty much assured with the 1D3 / 1Ds3 by virtue of the dual Digic processors.
Or put another way,
"if you never fill the buffer when shooting then it won't matter what speed card you use" and "if you do fill the buffer then you'll benefit from a faster card up to the point where it exceeds the camera / card interface speed" - which at this point appears to be (give or take 1%) a maximum of 30MB/s - which is Extreme III territory, not Extreme IV territory (Keep in mind too that Dave is only shooting with a 20D - which (off memory) transfers around the 6MB/s mark max).
Did I miss anything anywhere?