It used to be more spread out with a strong valley presence, but over the past couple years they've been force reloing people to Arkansas. It really started to ramp up at about the time I left
From my datalink, above, it seems the SFBay (Sunnyvale / Castro Valley / &al) has comparable H-1Bs to Arkansas — but these employees are spread all over The States. Not sure about citizen workforce, but I'd recon' they're similarly proportioned.