There are stacks of sports and press photographers who choose to use Canon off their own back, the 1D MkIIn was the best auto focussing digital back ever seen bar none, it was lightyears ahead of the D2x/D2H. All of Canons Telephotos had IS way before most Nikon lenses too, which is a big thing.
Everyone knows, that until recently Canon were well head with the Pro DSLR stuff, the D3 is changing that.
There aren't really any other companies selling telephoto sports/press stuff which competes with Canon/Nikon, Pentax lost their way with the pro stuff a long time ago, and most of their big lenses are mile behind Canon/Nikons. Take the pentax 600 F4, totally awful lens, 13lb, no IS, slow AF and soft at F4.
I don't think its fair to assume that press/sports photographers all use Canon because they're paid/given free kit, I know several football shooters who all use 1DMkIIn and have nothing to do with Canon, because its the best focussing body you can buy for sports, some of them are considering the D3 because its also a cracking camera, but they do so at their own will.
Its all just kit at the end of the day anyways, most DSLRs are P&S. Unlike my friend who takes 30 mins setting up his medium format camera
I have sleepless nights worrying what the light/weather will be like, not whether my lenses are sharp lol.