I agree there is this trend (the hip Social Media whatever startups are loss prominent in SF), but you do see some lower-level silicon valley startups in SF. Salesforce, Riverbed Technology, BitTorrent, Heroku, Dropbox, Square - just to name a few known ones. Admittedly there is few electronics stuff here (outside of biotech).

That said the Bay Area definitely has an OSI model going, with the lowest levels in the SE extreme (santa clara, san jose), higher levels NW (Mt View, Palo Alto), and software applications all the way to SF.