Case(s) in point: if, indeed, the core of cities is the most energy-effective place for people to settle, how and when will we re-inhabit American cities that are being abandoned by the jobless and foreclosed upon?

The combination of job losses and free-falling property values is sending many American urban neighborhoods back to zero, that downward spiraling state of neglect and entropy from which they will need to figure out how to start all over again, resettling abandoned residential areas and rebuilding abandoned infrastructure.

I can imagine the situation getting dire enough that people will be drawn to the giveaway housing of downtown Detroit as affordable and potentially efficient domicile. But the social resettlement requires a plan just as much as the physical resettlement. How do urban planners approach such a challenge?

As reported on in Forbes, a mass exodus taking place from some cities, with vacancy rates spiking.

Now, asking prices for rentals are down 10% from July, according to the Real Estate Group New York, a research firm, and people are packing up: 373,364 residents have left in the last year, according to the Internal Revenue Service, a net loss of 80,000. Unemployment is at 7.8%, above the national average of 7.2%, thanks to a 10% December drop in financial services jobs. The U.S. Conference of Mayors estimates New York will lose 181,000 jobs in 2009.