To everyone who voted new engine:
Do you mean improvements to the current Source engine, or a brand new engine? Because you do realise, Valve has never written a brand new engine before?
The original Source engine was a heavily improved version of the engine from the original Half-Life, which was an improved version of the original Quake engine. Since HL2 they have added HDR, a new particle system, hardware-accellerated facial animation, multi-core processor support, an OpenGL renderer and much more. There is nothing to stop them continuing to update Source to maintain competetiveness with their rivals.
Perhaps they have stacked up a batch of new engine features to be revealed in one massive update, but that's not the same thing as making a brand new engine. There wouldn't be any point to that, it's just too massive of an undertaking and wouldn't be any better than just updating their current engine.