The employees work on the projects they want to work on. Portal 2 happened because Portal 1 came out, and a lot of people in Valve wanted to do a Portal game. I would've be surprised if there is a small number of people working on the early stages of Half Life 3, or if there is a lot of people working on it without any announcement. It doesn't seem like Dota 2 would take all of Valve's development resources. Granted they're doing steam box stuff, but I imagine that a large portion of the studio is working on something we haven't heard about yet.