Yes, apparently it's a trilogy now. The first expansion episode of Half Life 2 comes in less than a week -- June 1. Now Valve is saying that there will be two episodes to follow: one by the end of 2006, and a third to round out the trilogy by Christmas 2007. I don't know if this represents one story arc or whether that really is all the episodes they have planned for now. What comes next will undoubtedly be based in part on the success of these three.

Source: Next Generation