Gabe Newell of Valve Software met with a bunch of folks from 4chan recently and the topic of games came up, of course. Now, my theory on why we haven't seen a Half-Life 2 Episode 3 is that I think they are waiting to release Half-Life 3 with a brand new Source engine.
Well, we didn't get that confirmed, but we did get confirmation that they are indeed working on a new engine and waiting for a game to showcase it. They've been working on it for a while now so my hunch is we'll hear something about it soon with the announcement of the next Half-Life game.
The video is linked below and the quality isn't that good, but it's another little piece of the pie to what's coming out in the future for Valve Software.