Until recently the answer was very clear. Canon utterly dominated Nikon when it came to what the pros used for their sports photography equipment. You could always tell by the looking at the white Canon L-Series lenses on the sidelines of games. But check out this photos from the Beijing Olympics last year. There are quite a few non-white lenses attached to camera bodies. Those black lenses are attached to Nikon cameras. I'm sure this is due to the introduction of the Nikon D3 digital SLR.
By the way this photo is from a Russian site that contains some of the best sports photos from the Olympics I've ever seen.
