Valve confirms HL2: Episode 2 on Computer and Video Games offers the news that Valve is indeed working on another Half-Life 2 episode to follow their upcoming release. This is actually a no-brainer after the first follow-up was renamed Episode One, but the story, based on a yet unpublished interview with Valve's Robin Walker, does shed some light on what these upcoming episodes will be like:

Valve's Robin Walker, designer/engineer on Episode 1, informed us that Episode 2 "has been in development for some time" in an interview that'll be winging its way to you shortly. Although he didn't elaborate further, it sounds as though Episode 2 may appear sooner than we could have hoped. We're definitely keeping trigger fingers crossed on that one, especially as the developer has aimed to offer "between 4 to 6 hours" of gameplay time with Episode 1, according to Walker - shorter, perhaps, than fans might have expected.