I appears that researchers at QUT’s Programming Languages and Systems group are working on an implementation of Ruby for .NET. The “secret” is reveal on this page. The work is support by Microsoft Research. Davis Thomas also mentions it in an article about the emergence of agile languages. There seem to be no other sources of information at present.
QUT is my old university and John Gough my was my favorite lecturer. I studied compiler construction under John and later wrote a small compiler under his supervision in C# targetting .NET (and using his book Compiling for the .NET CLR as a resource). It’s a small world. I’m working in the UK at the moment. The guy who sits next to me at work comes from South Africa. He studied under Pat Terry at Rhodes University. Pat is also a compiler guy. John and him were involved on the Modula-2 language commitee. Turn’s out that I used Pat’s Coco/R (C# version) on that project.