Richard Florida, who got famous creating the "creative class ," has a new series of maps out charting class structure in American cities -- not on the basis of income or wealth but on the type of work people do. Sfist has a nice copy of the San Francisco version here.  It shows, on the surface, that this city has virtually no "working class," some "service class" and lots of "creative class."
Overall, it's a picture of a city in the late stages of terminal gentrification -- but it's also a bit misleading.
San Francisco long ago lost much of it's traditional blue-collar work -- manufacturing, production, distribution, and repair -- although there's still some left. What we don't have is a lot of unionized blue-collar jobs (like the Port of Oakland offers). That's pretty clear.
But unionized jobs that don't require advanced degrees still exist in San Francisco -- they're just in the public sector. I suppose Muni drivers get defined as "service class" by Florida, but that's really not accurate.
Nor is the notion that "creative class" people all make a lot of money. I suppose there are artists and musicians who are getting rich in San Francisco, but I don't know any of them.
If anything, Florida's approach just underscores the changes in the American economy in the past few decades. It doesn't do much to help understand how the actual demographics of the city have changed, how wealth has become more concentrated and poverty more dire. So I don't really get the point.