Microsoft begs to differ:
They have a new OS they are working on from the ground up, 100% orriginal code. It's called "Singularity", and it's kernal has been sucessfully coded in C# (actually, a derrivitive of C# called sing#, but it's still C#
)
it's actually a MicroKernal, and is based on giving more security.
it's an interesting topic
http://research.microsoft.com/os/singularity/
http://www.microsoft-watch.com/article2/0,2180,1882174,00.asp
what do you guys think?
[edit]
here's more:
on page 31 of this PDF: ftp://ftp.research.microsoft.com/pub/tr/TR-2005-13 5.pdf they have this chart: