In an informal interview with Valve while a few 4Chan members were touring Valve's Bellevue offices, Gabe Newell was asked about a new game engine and his response was "we've been working on new engine stuff for awhile." Details though aren't clear at this time whether it's a "Source 2" engine or something radically different.
Gabe didn't comment on Half-Life 3 directly during this 4Chan Q&A, but it wouldn't be surprising if any new Valve game engine would be introduced in the widely-anticipated next game release.
Hopefully this future Valve game engine will contain additional Linux optimizations. Right now with the Source Engine on Linux they are using an abstraction layer since Source has been targeting Direct3D for rendering while on Linux and Mac OS X they need OpenGL. The performance results are good for Source Engine on Linux, but any further performance improvements would certainly be welcome.
If the future Valve game engine settled down with OpenGL, SDL, etc on all platforms rather than Microsoft DirectX, that would be a win and would force GPU hardware vendors to further enhance their GL stacks. Bettering the Linux support is also a likely bet since a Steam gaming console warrants first-rate Linux support.
Anyhow, we will see what else emerges about Valve's future game engine in the coming months. Right now Valve's busy with the Steam Linux Beta. Below is the 4Chan video in full. Also see (if you haven't read already) the Valve's Gabe Newell Talks Linux Steam Client, Source Engine and A Special Linux Delivery At Valve Software from my trip to their Bellevue offices earlier this year when delivering the exclusive Linux news months ahead of the official announcements.