"There goes the neighborhood." Every so often that cry goes up in San Francisco, announcing a new chapter in American cultural history, as the rest of the country looks on. There were the beats in North Beach, then the hippies in the Haight, then the gays in the Castro. Now it's the turn of the techies who are pouring into my own Mission neighborhood, among other places. Only this time around, the green stuff that's perfuming the air is money, not weed.