Valve's Gabe Newell Basically Confirms New Source Engine
Valve's co-founder Gabe Newell got a visit from the Internet's /v/ board from 4chan.com, and they asked him what's going on with the Source engine.
You can see video of the visit below, where Newell preemptively knocks down any Half-Life 3 questions, but he does talk about additions to the Source engine. The audio is a little garbled, but Newell says that Valve has been working on new engine stuff, but they're waiting to show it off until they have a new game. Someone then asked, "Is it more than just an extension to Source? Like, is it an entirely new engine?" To which Newell replied, "Yeah."
There you have it! A new Source engine is coming at some point, and if it is related to Half-Life 3, Gabe Newell would never openly admit it.