Played a bit with a 40D.
In short it's a great camera. A major strength is burst speed, especially when used with the more costly Canon zooms for action sequences. The handling and ergonomics are great; the camera falls into the hand "naturally" and although not small or light, it "shrinks" as you use it and stays out of the way, allowing you to just shoot pictures. I also liked the 3 separate "memories" that allow you to pre-programme the camera however you like, then set that "state" into memory so that you can easily recreate those exact settings for a particular circumstance (example: an early AM dusty rallye stage from 100m out and into the sun will require different ISO and white balance settings than a bride in a white wedding dress holding a red rose while standing on a white ski slope -- and best of luck with either scenario...). In any case, this is a great body for a sporting event.
The live view LCD mode is also useful. I was surprised at this frankly, since I've never used that mode with an SLR. However, it's really handy especially when the camera is on a tripod and one is using a large focal length lens to shoot a faraway object. In this case, live view helps spot the "target" much easier than the viewfinder. This would be interesting to amateur astronomers etc. Interesting point: live view is not as useful for sporting events, as it does not seem that the auto focus functions in live view as it does normally -- an owner can correct me here, but I don't see how to use live view on the 40D to hold the camera high overhead (so as to shoot over a crowd of people) and get accurate focusing on a rapidly moving target.
No matter, I'm nitpicking. If you have the money you owe it to yourself to at least try the camera out. I'd prefer it over the Nikon D200, as it feels "nicer" and better assembled IMO. I don't know about the D300 though.