They announced DOTA 2 because it has a long and lengthy beta period that requires input from "pro" players in terms of balance and whatnot. L4D1 was a purchased property announced by Turtle Rock.
We don't know what the actual Half-Life team is working on but presumably it's Half-Life 3 given the silence and refusal to comment from Valve. I also doubt there will be an actual third episode anymore, I think they'll just roll it into Half-Life 3 somehow. I'm more interested in seeing what they're doing with the latest iteration of the Source engine more than anything else.