Nemesis: Fine fan-boy. Here are your facts:
Lens test of Sigma 70-200 f/2.8 HSM vs Canon 70-200 IS f/2.8. You can click around to choose different combinations of aperture and focal length of the two lenses and decide for yourself if the Canon is any better (much less $700 better).
OP: With a 40D that goes from ISO 100-1600, I would probably go with the lens that gave me a 1-stop advantage.... especially if the image quality was the same and the prices were similar.
I've used my Sigma 70-200 indoors for ice hockey and broomball photos and I had the ISO as low as I dared at 1600 with the lens wide open while still exposing to the left.
I could have shot at ISO 3200 with the f/4 if I had to, but a 40D would need a flash, or expanded ISO (bad idea unless you like noise), or a different lens to get those photos.
Again, there is nothing wrong with the 70-200 f/4, but i guarantee that you'll end up wishing that you had the extra stop when you go inside.