http://en.wikipedia.org/wiki/Pareto_distributionIt seems, for reasons unknown to us, that a great many aspects of human (and natural) activity follow a power law. (more on power laws:
http://en.wikipedia.org/wiki/Power_law) In brief a power law describes phenomena where certain proportions seem to hold true regardless of scale.
A Pareto distribution (named after an Italian economist) is sometimes known as the '80-20' rule - the fact that in any economy, the tendency is for 80% of the wealth to be controlled by only 20% of the people. Curiously, the same appears to hold true for things as diverse as city sizes, grains of sand on a beach, and so on. That is, (approximately) 80% of the world's population lives in 20% of the world's cities, 80% of the mass of a pile of sand resides in 20% of the sand grains (the other 80% of the grains being increasingly smaller) and so on.
So it appears that disparities in wealth are in fact natural, although the
degree of disparity may or may not be sustainable in a given economy. You should also investigate the Gini coefficient, which is a measure of wealth distribution (by another Italian economist, oddly enough). scroll down past the mathematical bit if you're not math inclined, there's a lot of interesting general information after that:
http://en.wikipedia.org/wiki/Gini_coefficient