Firstly, I disagree that games are what drive technological advancements in the PC and operating system software insustry.
Longhorn is not built on .NET - it has just replaced two of the three core windows dlls - user32 and gui32 with the managed Avalon presentation layer, and lots of communications stuff too.
Considering that games don't use these two anyway (sure, they do, but only as intensively as Notepad does) and they use C++ and DirectX, I don't see that it'll make a blind bit of difference.