In regards to Episode Three, Portal's sequel, etc, check it out at CVG (here's a select excerpt):

You guys are taking your time with the Half-Life episodes, too. How's Episode Three coming along?

Lombardi: Well, the gap between Half-Life,Half Life 2 was six years. It's not quite four years since then and we've already released two follow-ups that we're really proud of. We didn't milk the cow, so to speak, and pump out more of the same content.

While the word 'Episodic' conjures up this idea of TV where episodes are aired every week, maybe that's not the best term to use for this. I do think that we've hit upon something that allows us to have a more enjoyable development experience - to spend six years on the same game is kind of a death march.

We've hit with episodes around every 14-16 months. It won't be another six years until you see Freeman, but it won't be next week. I think we're improving our ability to produce interesting new content in a more timely fashion.

Maybe it won't be as long as Half-Life 2, but hopefully it'll be just as good and just as innovative.