Seattle has always modeled itself on San Francisco, and now both cities are having doubts about the model. The California city picked itself up after the dot-com bust by pursuing condos, fine restaurants, young residents, and the new trends in the economy such as biotechnology, green tech, and stem-cell research.

Sound familiar? It's a formula for Seattle and Portland. But now the doubts are setting in. Minority populations are moving out, particularly Blacks. They are becoming cities with few children and few families, in effect cities without a middle class. The prospect is a city with extreme wealth and pockets of poverty.

A valuable survey of San Francisco's rapid march up the income scale, in the San Francisco Chronicle, worries that the city will be unrecognizable in the next five years, in effect turning from a real city into a boutique city. Corey Cook, assistant professor of politics at the University of San Francisco, points out the dramatic shifts in demographics:

He sees San Francisco attracting 25-year-olds, "fresh out of MIT, who want to come to San Francisco because it is the coolest place in the world."

It could become a city of the young - and you can see that trend now in the clubs, in the hip restaurants. Ten years later, this scenario goes, these San Franciscans want to raise families, but the housing stock now going up is one- and two-bedroom condos, and the older houses, their prices driven up by scarcity, are unaffordable. Even now, even in a mortgage crisis, single-family homes in desirable neighborhoods like Noe Valley sell for well over $1 million.

So, Cook said, these San Franciscans move to the suburbs. It's an old story in cities: It happenedin North Beach, the Sunset, the Richmond and other places.

But in the new scenario, after years go by and their family is grown, these ex-San Franciscans move back - a movement planners call "new urbanism."

But now there are places for them like One Rincon Hill, the Millennium Towers and Soma Grand, which advertises itself as "a boutique condominium development."

The new, older San Franciscans who live in them support the opera, the theater, the symphony. All three were booming in 2007, a year in which the San Francisco Opera even showed a profit.

In this scenario, not long in the future, what has developed is a city of the young and the old. "If this happens," Corey said, "you definitely have a different kind of a city."

Can these trends be reversed? The article notes that the Board of Supervisors leans firmly left and in favor of condo development (to stop sprawl), but Mayor Gavin Newsome is a moderate, more aware of the backlash.

One thing is likely to continue the same. San Francisco is "always changing, but it opposes change."