The econ-o-sphere is all a twitter about this report from Credit Suisse on urbanization trends, I found it through The Economist but there is a long link chain that precedes thins one.
Anyway, as a development economist, I know that one of the seemingly inexorable trends is urbanization. This makes sense, as a low-income economy develops, what starts as a largely rural agrarian society characterized by small farms for self-consumption and local market exchange becomes more advanced, more mechanized and more industrial. Industry requires scale and concentration and thus more and more people start to locate together. What is less obvious, perhaps, is all of the ancillary benefits to this congregation: the efficient provision of goods and services, the knowledge economies or clusters (which to an economist is just positive externalities) and so on. So prosperity and density go hand in hand.
Even within the US this trend can be seen:
The good news in all of this is that density is efficient and in many ways holds the key top the future of humans on this earth.