The folks at Microsoft Research have been working on a "research" operating system they call Singularity.
So what's so cool about this?
Singularity turns current preconceptions of operating systems upside-down. 95% of the microkernel is written in a variant of C# called Sing# (the rest is assembly and C). They use a native compiler to turn that MSIL into real bytecode. So there isn't a "CLR", at least not in the kernel.
The concepts of Processes, Threads, Drivers, and Communications is totally up-ended as well. Everything in Singularity runs in a "SIP" (software isolated process)... applications, services, even drivers. These SIPs all run close to the metal in ring-0, cannot reference memory addresses in the kernel or any other SIP, and are required to be verifiably safe. SIPs are only allowed to communicate with each other via something called the "ExchangeHeap", and then only by tight message/protocol contracts. A side effect of the way they forced the strong-typed "owned messages" is that certain data transfers that were traditionally heavy on bitcopies are now zero-copy operations. They also defined some S# language extensions to define these message patterns - it's pretty slick stuff really.
You can read the entire project overview whitepaper to find out all the other cool stuff they managed to do in this Concept OS.
I read the paper, and was totally expecting the cost of executing everything in isolated SIPs to be a performance dog. I was shocked to see that in actuality, the performance was equivalent (or in some cases phenomenally better) than other *tweaked* modern operating systems.
Anyways, this is some cool research. I suspect parts of this work will wind up in future OS products - I recall the CLR team was reluctant to discuss the ideas of "hardware CLR" - perhaps they are further along in that regard than they are leading people to believe...