You can get a SIM-card from T-Mobile USA, coverage in the Bay Area is generally decent. They run GSM and UMTS networks in most parts of the Bay Area, on standard North American frequencies (you should have a "world" phone to connect), and T-Mo's prepaid SIM-cards come with voice roaming free of charge (but no data roaming at all), in case you get out of the Bay Area and there's no native coverage otherwise.
I think there are only two GSM/UMTS networks in the Silicon Valley: AT&T and T-Mobile USA; but there are also many companies that do MVNO. There's also Sprint, Verizon and MetroPCS, which run non-GSM networks, and they also have many MVNOs (other than MetroPCS, perhaps). Best bet is go to Best Buy, Target or Walmart, and get whatever will fit your needs.