Technically SFO is one of the very few airports (and the largest one I believe) that contracts out its airport security to a private company. The company and its operations are regulated by TSA, and for all practical purposes offers the same experience as any other airport, but the people who look at your bags and such are not TSA employees.

See: http://www.flysfo.com/about-sfo/safety-security and http://blog.tsa.gov/2010/11/airports-who-opt-out-of-tsa-scre...