Following this discussion in another thread and several people saying that to shoot a fast moving subject in the distance, you can just prefocus on infinity, and my insistence otherwise and several people disagreeing, I took a few shots in between overs at the Durham v Glamorgan game this afternoon. Don't worry, you're not meant to understand that. About the only person with a decent chance would be Ian *wink*
This is the full frame version of the shot:
400mm lens, 1.5x focal length multiplier, f5.6
Identical levels adjustment and unsharp masking applied to all the crops. Sorry for the levels of grain involved, light wasn't great and I was shooting with a fairly high EI.
This shot was taken with visual focus through the viewfinder on the power cables/structure in the distance.
This shot was taken with visual focus through the viewfinder on the tree immediately to the right of the power cables/structure in the distance. By my estimation this was about 3-400m or so away from the camera.
This shot was taken with visual focus through the viewfinder on the light coloured tree that's in focus. By my estimation this was about 250-300m away from the camera.
This shot was taken with the lens preset as close as possible to the infinity marking on the lens barrel. Bearing in mind that lenses expand and contract, the temperature was about 20 degrees and the sun was out.
Now, from close comparison, the following is apparent to me.
 Focus is different in all four pictures. Look carefully, even between 1, 2 and 4. 1 is farthest out, 4 seems to be between 1 and 2, and 2 is the closest (obviously 3 is closer).
 This is all already virtually infinity. The closest tree was at least a good 250m away.
 A 1mm turn of the focusing ring either side of infinity completely throws the focus off that point.
 Focus at "infinity" on the barrel did not result in the sharpest picture of the furthest object.
 An F16 shot to the same composition as Mylau's shot would have been about the distance of the mid-distance in the overall shot that I've displayed here (look at the size of the people in the crowd as reference). Considering they are nearer than the nearest tree, it should be clear that accurate focusing is needed. See how out of focus the nearest tree looks in the picture with the focus furthest away.
Bottom line, infinity focus doesn't work for a lot of stuff once you hit reasonable telephoto range. Or you stop down a stupid amount. In the case of Mylau's picture, infinity focus would not have worked.