I came across the truck on Mission Street across from Yerba Buena Gardens on Saturday around 1 pm...5 minutes after I ate lunch. I saw several people eating from open pizza boxes but couldn't get a decent look at the product. The price I liked, though -- $7 per pizza (their website says $10). Two varieties were on offer.
They were at Sydney Walton Square today according to their twitter (I don't even know where that is).