Valve have hinted at a revamped Source in the past. I think it may have been in The Final Hours of Portal 2. Basically, they've had a number of programmers working on it for quite a long time now and the underlying changes are drastic enough that it's going to split the code base. Also, for what it's worth, during one of many interviews with Wolpaw, he implied in his characteristically coy manner that data streaming is in.
Edit: Oh, and when Valve recently did some site redesign, they added
this:
Valvesoftware.com said:
In addition to Steam, there is Source, our own state-of-the-art game engine. Its sophisticated character animation, advanced AI, real-world physics, shader-based rendering, and super extensibility have helped us create some of the most popular and good-looking games on the market for computer and consoles. We don’t like to brag, but Source is considered the most flexible, comprehensive, and powerful game development environment out there. And it’s about to get even better.
Of course, who knows how long "about to" is in Valve Time.