a lot of the big lenses the sports shooters seem to use are f/2.8 lenses and I'd guess in that environment they'd probably just have the cameras on full auto, though with the aperture wide open, you blur out the crowd/background nicely ... when I went to a tahs game earlier this year, one of the media photographers came over and asked if my camera was a canon. I said 'yep' and he said 'you reckon it'll fit on this?' (his 100-400mm, f/2.8 lens). It fit. It was awesome.
here's a cop girl that I got a photo of after the game when I was playing with his lens. they were trying to get her number, and I promised them a photo ... 400mm, f/2.8:
