The smell itself wasn't poisonous or anything (beyond to the nose, of course) -- it turns out the pipe, which hasn't been used since the 1940s, had retained, along with some stale water, the bad smell they add to natural gas so you know when there's been a leak. Mmmmm. No natural gas was actually released -- though a friend of SFist on the scene reports that a delicious-looking foam started coming out of the ground.

Poor PG&E workers, right there when the smell came out! They ended up filling the pipe with sand as fast as they could to kill the odor. A vacuum truck will be coming by next to clean out all that sand so they can get back to work.

Picture of the scene from ABC 7.