Glenn Reynolds links to San Francisco Chronicle staff writer James Temple, who describes "urban flight flipped on its head":

The number of low- and middle-income residents in San Francisco is shrinking as the wealthy population swells, a trend most experts attribute to the city's exorbitant housing costs.

Many worry it's increasingly turning San Francisco into an enclave of the rich, where nurses, firefighters, cops, teachers and other professionals aspiring toward homeownership or in need of cheaper rent can no longer afford to stay.

"A kind of derogatory term for the city would be Disneyland for yuppies," said Hans Johnson, demographer with the Public Policy Institute of California. "There is a legitimate public policy concern when a city that many people have lived in for many years and regard as their homes becomes so expensive they can't afford to live there anymore."

Last year, USA Today noted, “San Francisco Hopes To Reverse Black Flight”, but it's part of a much larger trend, as I noted earlier in 2007:

As a city, San Francisco has had its share of problems in the 21st century, among them: declining population, declining economy, declining children, contempt of the US military, a large and often militant vagrant class, and declining tourism.

Fortunately, when it comes to making an effort to solve those problems, local government has its priorities firmly in order.

But it's not just San Francisco--when I interviewed Steven Malanga of City Journal a couple of years ago, he noted the same trend of the middle class being squeezed out, leaving only the wealthy and poor occurring in New Jersey as well. And I imagine anywhere there are punitive liberal policies simultaneously raising taxes and making a difficult environment for new housing and entrepreneurship, this trend will occur, but San Francisco's certainly had a multiple-decade head start. If it's entered the endgame, perhaps it will serve as a warning to other locales, and not as a prototype.