San Francisco Bay Area real estate prices - from the 2007 peak through 2009
This chart from Dana Kuntz of Pacific Union
The article referred to in his notes is here.
San Francisco dropped the least, but 20% is significant.
Socketsite uses Case-Shiller data which doesn't include Napa and Sonoma, and does include San Mateo which is not in the above chart. That 5 county chart shows the Bay Area down 27%. But what is most interesting in the SocketSite post is the 3rd chart down - here is that SocketSite chart:
Notice that San Francisco did not rise as much as Los Angeles and New York. I'm quite sure the City of San Francisco didn't rise as much as Contra Costa and the other counties either. This is important because the bigger you are.... or the higher they rose, the farther they will fall. So don't expect City of San Francisco prices to drop any where near as far as other Bay Area counties. That said, you probably can expect certain parts of San Francisco to fall quite far - like SOMA/South Beach which were severely over priced.