Seems that San Francisco is a hard city to live in for young middle class families, so they're all planning on moving (or at least half of them, according to one study). If you're one of those cranky bastards whose favorite coffee shop has been overrun by kids, sounds like you could move to SF and just wait'em out.
If, on the other hand, you're a young/new dad, and you're staring at a future of $750K shoeboxes, no parking, and expensive preschool tuitions, well... beats me. New York City's no suburban paradise, either, but that's why we love it. NPR's got a story on the topic. Check it out.
San Francisco: A Childless Future? [npr via dt reader eric]