Over at NRDC's Switchboard blog, Kaid Benfield has a couple of posts lamenting the fact that the environmental movement devotes relatively scant attention to issues like urban redevelopment or inner-city schools. Those sorts of topics may sound far afield, but they're really not. Boosting the number of people who live in inner cities (or even in denser suburbs) is an incredibly green thing to do—reducing sprawl, curbing vehicle emissions, protecting rural watersheds, improving public health… But environmentalists don't tend to be nearly as engaged in, for instance, local meetings on revitalizing inner-city neighborhoods as they could be.

Meanwhile there's the education angle: Many families with children aren't going to want to live in urban centers as long as schools are in poor shape. Though it's worth noting that this isn't a strict obstacle to denser living. As Christopher Leinberger has pointed out, 88 percent of U.S. household growth through 2040 will be singles and childless couples, and many of those people would much prefer to live in walkable, urban areas—the big obstacle here isn't schools, it's the supply of available, accessible urban housing.