On a related note, don't forget to visit the Computer History Museum if you're in the area (might be a good alternative if your Google plans fall through) Hope they still have the difference engine on display.
I've interviewed at Google a few times and eat lunch there every week or two. Great campus, but not exactly something that I think people should push for touring when they're in SV.
A few other awesome nerd things to see/do:
* Winchester Mystery House http://www.winchestermysteryhouse.com/index.cfm
* California Academy of Sciences http://www.calacademy.org/
* USS Hornet http://www.uss-hornet.org/
* Fry's http://www.frys.com/
Or visit Mozilla (I work there ;) )