Don, if you haven't read the Canon whitepaper on the 5D, you should do so. You can find a copy at http://www.robgalbraith.com/public_files/C...White_Paper.pdf
Here is a relevant quote from the whitepaper re focussing speed:
"Predictive AF can focus track a subject approaching at 186 mph up to 66 feet away with an EF 300mm f/2.8L IS USM lens. The EOS 5D camera uses the same statistical prediction computation as the EOS-1Ds Mark II camera, incorporating data of past focusing operations. Because it can repeat more focusing operations in a short length of time, the predictive AF control can operate effectively from the first shot, even if a subject is moving erratically or if the subject’s movement changes just before shutter release. (In this case, as with the EOS-1D Mark II camera, when focusing is possible, the lens drive is executed based on the focusing result right before shutter release.) For still subjects, too, AI SERVO AF is a useful option. Focus control is exceptionally stable; the lens does not move, even minutely. If the subject should move unexpectedly, focus detection is always active to enable subject tracking."