In keeping with this week's class warfare theme, there's a fascinating article in this month's Atlantic Monthly by Christopher B. Leinberger that posits that today's exurban McMansion communities are tomorrow's slums.
The gist of Leinberger's argument is that we are turning back to a preference for more urban living, which means that city centers and older, more affluent "railroad suburbs" (those with actual town centers built around a commuter train station) are going to continue to become hot; while isolated McMansion communities with lots of cul-de-sacs but few amenities, are on their way south. Leinberger's prediction is that as owners are unable to make their money back on their (shoddily built) McMansions, they will take to either abandoning them or subdividing and renting them, creating isolated ghettos far from the notice of the chattering classes.
In other words, American cities are all going to become a lot more like Paris, where the lower classes are isolated in housing projects in outlying banlieues.
There are myriad implications for marketers if this shift happens, both in terms of the type of products desired by people in urban settings who rely on public transportation and in terms of the actual urban settings themselves, where people are able to actually walk places. Off the top of my head, that means greater emphasis on outdoor advertising (bus shelters and train station posters) and word-of-mouth.
Since the first part of the equation-- the wholescale gentrification of urban areas-- is already well under way (check out the number of poor people in Manhattan or San Francisco today vs. ten years ago) Leinberger may just be on to something.