Here it is: http://research.microsoft.com/os/singularity/
There are several interesting ideas in this new operating system. It's not just an "OS written in managed code". There are a new set of techniques in modern software development: contracts, verifiability and static analysis. Singularity combines them in an elegant framework, and applies these notions in the operating system development. New areas that benefit from these new techniques are, for example, writing device drivers.
P.S. There is also a technical report here - a must-read for anyone interested in operating system research.