That is not the case, as far as I know (see also http://en.wikipedia.org/wiki/Development_of_Windows_Vista).
MS have written at least one OS on top of the CLR -- Singularity. Very cool stuff, albeit a research system.
Singularity is indeed fascinating, but it does remain a research OS. I hope something like it finally comes to light in a future release.
WP7 might have been a good breeding ground for such technology, but even that never managed to shake off it's own legacy OS underpinnings.
That is not the case, as far as I know (see also http://en.wikipedia.org/wiki/Development_of_Windows_Vista).
MS have written at least one OS on top of the CLR -- Singularity. Very cool stuff, albeit a research system.