Just an FYI to everyone coming from out of town for the concert in Golden Gate Park. Sunday September 12 is also the San Francisco T-Mobile International, a bicycle race through the city of San Francisco. Last year the event had an extimated 600,000 spectators. Lance Armstrong will probably be racing in the event again this year, and given that he's likely to win the Tour de France again in a couple days, interest in this year's race should be just as high as last year's. Many city streets are closed for this event, and combined with all the extra people in town, traffic is a huge MESS.
Fortunately the area most affected will be the eastern/downtown section of the city. Golden Gate Park is on the western side. I would definitely recommend avoiding Union Square/Downtown/Fisherman's Wharf hotels. Plan on taking public transportation to the show, and plan on taking it home to your hotel. If you don't know how to do that, there's a cool website called
http://transit.511.org that can help.