In 2008, the world reaches an invisible but momentous milestone: For the first time in history, more than half its human population, 3.3 billion people, will be living in urban areas. By 2030, this is expected to swell to almost 5 billion. Many of the new urbanites will be poor. Their future, the future of cities in developing countries, the future of humanity itself, all depend very much on decisions made now in preparation for this growth.
Most of the urban growth cited by the article is in developing countries. This is because they are beginning to industrialize and make things. America is not making as much any more and most people work white collar jobs. There is just not as much of a reason to conglomerate around a place of work like there is if the whole town works building cars, for example.
(To put up a disclaimer, I have just extrapolating this from what I think. I have not done a significant amount of research. There is a lot science fiction talking about this however, such as Daemon, Freedom TM, and even The Stars My Destination from way back. I also found this article that has some facts, despite being a little old. http://www.innserendipity.com/ruralren/rebound.html)
From http://www.unfpa.org/swp/2007/english/introduction.html
Quote from the introduction
In 2008, the world reaches an invisible but momentous milestone: For the first time in history, more than half its human population, 3.3 billion people, will be living in urban areas. By 2030, this is expected to swell to almost 5 billion. Many of the new urbanites will be poor. Their future, the future of cities in developing countries, the future of humanity itself, all depend very much on decisions made now in preparation for this growth.