Indeed. It is the Singularity Research OS. Links: http://en.wikipedia.org/wiki/Singularity_(operating_system)

http://research.microsoft.com/en-us/projects/singularity/

Note that managed code is just one feature of Singularity. They have many other important concepts (like SIPs).

Plan9 ain't bad either. There's also different C# clones (that aren't based on Singularity)

IIRC, though you write in a managed language, it's jitted at install time so it doesn't run under a managed runtime.