From the UStream site:
Look inside the clean room at NASA's Jet Propulsion Laboratory in Pasadena, Calif., to watch the next Mars rover being built. The camera is located in a viewing gallery above the clean room floor. There is no audio on this video feed.
As of Oct. 2010, technicians are working from approximately 8 a.m. to 11:00 p.m. PDT Monday through Friday. The camera shows a portion of the clean room that is typically active; but the rover, spacecraft components and technicians may move out of view as work shifts to other areas of the room. When activity takes place in other testing facilities around JPL, the clean room may be empty. The camera also may be turned off periodically for maintenance or technical issues. We will update this page often to inform viewers of these situations.