Update: Doug Lombardi has responded to Joystiq, and he said that this is more of a case of Valve hiring two programmers, not necessarily to do with Valve opening a whole new studio.
"We got a response from Valve's Doug Lombardi, who did confirm that Valve hired both Tod Semple and Jeff Gates; however, he did wish to clarify this doesn't necessarily mean Valve is opening up an office in San Francisco, despite what Semple's LinkedIn profile says. 'As a point of clarification, this is probably better categorized as Valve hiring two new employees instead of an acquisition of a company or opening of a Valve SF office'" (Joystiq).
"We are opening a new Valve office on the San Francisco peninsula," says a former PopCap programmer. Valve has acquired Star Filled Studios, a two-man development team that was founded by ex-PopCap and Blizzard employees, Jeff Gates and Tod Semple. According to Tod's LinkedIn profile, the acquisition happened sometime last November and Valve is currently in the process of expanding.
"My recent startup was acquired by Valve and we are opening a new office on the San Francisco peninsula."
This isn't the first acquisition in Valve's history. In early 2008 they acquired another California-based company, Turtle Rock Studios, and took over the development of Left 4 Dead. But that studio was eventually closed later that year and the employees were relocated to Valve HQ in Bellevue. TRS was reopened again in 2010.
There's no information about what Valve will be using their new San Francisco offices for or how large it will be initially, or anything like that. We've reached out to Valve for more information.
Thank you to our resident investigator Barnz for the information.