According to the blog, NVIDIA engineers have been "embedded at Valve" to help them out with the development of its Linux-based SteamOS platform. NVIDIA's engineers are helping to improve OpenGL performance, and optimizing overall performance for NVIDIA's GeForce GPUs. NVIDIA is also helping Valve port the Source engine over to Linux, as well as reduce input lag in SteamOS.
This shouldn't come as a surprise for most, with AMD securing contracts in all three next-gen consoles, NVIDIA needed to have done something: and this something is quite extraordinary. This is a big move for PC development, especially with NVIDIA and Valve working together. I think I've died and gone to heaven.