Yes, I'd say the 100-400, would be a very good lens to have trackside. The L glass is very highly rated and the IS is supposed to be a help as well especially with slower shutter speeds.
It might be a bit limiting in the paddock/pits though and you would end up stepping back a heck of a lot to get your subject in frame and possibly end up getting run over.
But I've learnt that I can acheive fairly reasonable quality in the pits with cheap n nasty lens.
Take a peep at this
http://www.f1-jimages.co.uk/2006/des...6/img_9122.jpg
taken with 18-55 that comes in kits with canon bodies.