I own both the Canon 100-400 f4.5/5.6 L IS and 70-200 f2.8 L IS lenses. I've used both on my 1Ds Mk II and 20D bodies to shoot my kids' sports (swimming, baseball, softball, etc.). I've also used the 70-200 with a 1.4X extender.
I find that the 100-400 is an adequate lens for this application. Turning up the ISO to 800 still provides relatively noise-free images on my 1Ds but allows me to capture the action with a faster shutter speed. The IS feature is also a big help as I don't use a tripod or monopod on the sidelines.
The 70-200 is a much sharper lens, but too short for most outdoor field sports. With the 1.4x extender, it's a little better, but the extender degrades the sharpness enough that I just settle on the 100-400 lens most of the time.
A redesign of the Canon 100-400 is long overdue. The push-pull zoom is outdated and at f4.5/5.6, the lens is a bit slow. Unfortunately, this is currently the only option in the Canon "L" series lens line up for this zoom range.
Bud James
North Wales, PA