I believe there is an obligation, in at least a very minimal sense (how far you go with it depends on your political perspective, I suppose). Maybe the first members of civilized society had an equal and clean slate and earned as much as they deserved, but its never been the case since then. Inequality is institutionalized in societies and I think its very important for us to recognize that people don't all start off on an equal footing; that individual success is not earned solely by one's talent/hard work. They are always extenuating societal forces which push us in the direction of prosperity, as well as poverty. Society should make some attempt to fix that disparity in opportunity. It'll never be perfectly equal, but that doesn't mean its morally acceptable to pretend as if institutionalized inequality doesn't exist.