Planners and others who wring their hands over urban sprawl find hope in a study that found 24 U.S. cities expected their downtown populations to grow by as much as 400 percent by the year 2010.

The study, prepared by the Brookings Institution and the Fannie Mae Foundation, found that newcomers preferred the inner city because it eliminates commutes, avoids traffic jams and provides a richer, more varied social life than can be found in the suburbs.

"The people moving into the central city," says Amy Liu, deputy director of Brookings' Center on Urban and Metropolitan Policy, "are smart, professionals with high incomes who create market demand in urban neighborhoods for grocery stores, high-end retailers, condos and other housing."

But as inner cities gentrify, the poor are being displaced, and it has created real friction in San Francisco and Washington.

"It raises questions about the role of the city," Liu says. "The city has always been thought of as home to all income levels and ethnic and racial types. But the new economy is pricing out people who thought the city was their home."