I don't know of any operating system besides lookingglass3d window manager, which isn't really an operating system but a window manager and X server variation, that is partly written in managed code.
All OS's that I know of are C with some C++ and a very small part of platform specific stuff in asm
I believe the entire "10 foot" UI of XP Media Center Edition is written in managed code (C# would be my guess as MC++ is so gnarly).
Given that one of the persistent rumors is that Longhorn will merge all the XP variants (MCE, Tablet, 64bit, etc.), it's likely at least a portion of the OS will be managed.
But it really depends on how you define "Operating System". It it (most) everything that comes on the Windows CD (probalby DVD for Longhorn)? Is it whatever comes from Microsoft that is pre-installed on a new computer? Or is it just the absolute bare essentials
required to support the Win32 API? If you go with the strict CS definition, then even if the Shell probably wouldn't count.
I'd bet lots of good money that Microsoft will ship managed code as part of the Longhorn deliverable. If nothing else, I'm sure the .NET framework will be installed with the OS and not be an optional component.