Well, according to the _extremely_ broken 'spot' rules in d20 an average character can't reasonably be expected to spot a man sitting on the ground in a field on a clear day at 100 yards.
Human vision isn't entirely motion based, but human scanning paterns are. In practice a person with good eyesight can spot a person at maybe at two miles or more if the person is moving and wearing a color that stands out against the background (or is backlit). That would be a spot check at a -1000 or more penalty under the rules, but well within most people's vision. Even a motionless human should be resolvable at a mileif you happen to be starting in the right direction. I've heard incidents of pilots with good vision picking up other aircraft at thirty or even fifty miles - well before thier radar resolved the object.
My solution is to put spot DC's on a semi-logrithmic scale. Up to 100' is a -1 penalty per 10' of distance. After 100', the penalty falls to -1 per 100'. After 1000', the penalty falls to -1 per 1000' and so forth. Same thing with listen checks. This allows a shout or a thunderclap to be heard at a considerable distance without having the sound suddenly disapate over the course of 200', and without having ludicrously low DC's to hear the sound like -1500 or something. Of course, such a rule probably doesn't interface well with epic level assumptions, but I don't much like epic level assumptions anyway.