As much as you might want to hear about Half-Life 3, or Episode 3, or... whatever, Valve's just not talking. That doesn't mean they don't have something else in the works. The recently announced Defense of the Ancients 2 will be getting a public beta soon, after Gamescom, Europe's biggest gaming convention.
Valve also told reporters at the show in Germany that DotA 2 is planned to launch at the end of this year, although it could very easily slip to 2012. You can register through Steam for the DotA 2 beta. If you're selected to participate, you'll receive an e-mail invite shortly afterwards.