Productivity Through Density
"Understanding the appeal of proximity - the economic advantages of agglomeration - helps make sense of the past and future of cities. If people still clustered together primarily to reduce the costs of moving manufactured goods, then cities would become increasingly irrelevant as transportation costs continue to decline.
If cities serve, as I believe, primarily, to connect people and enable them to learn from one another, than an increasingly information-intensive economy will only make urban density more valuable."
He cites a study that shows economic productivity increasing with metropolitan density, and looks at some of the reasons why.