The book actually says the throw/touch has to beat the runner to the base. However, in practice I do admit that I'm more likely to call on out if things are so close that I can not determine the difference.
I heard it explained once on a throw to a base - you see the foot hit the bag and hear the throw hit the glove simultaneously. Since the speed of light is faster than the speed of sound, the ball would have had to hit the glove before the foot hit the bag (not to mention that the pop of the glove happened some fraction of a second before the ball entered the mitt). Then again - if you calculate this out and are 10 feet from the bag, you're talking mini-microseconds.
