AF Tracking is an interesting subject.
Here's a thread about a test in a magazine using an approaching vehicle to test AF tracking:
Magazine Review: K20D/SDM AF.C performance vs. 8 competitors
Here's a thread that shows some AF speed tests (without taking tracking into consideration).
Disclaimer... I use a Sony A700 right now, so I may be a bit biased in that direction. ;-)
There should be a way to compare AF tracking in a wider variety of conditions, if you could take photos using multiple camera models in a controlled manner and calculate the number of keepers from a large group of images to see what works best for a given subject/movement type. It would be a lot of work to do it though (and you'd probably see a lot of variation based on subject type, lenses used, etc.).