That was just an experiment:
"Our goal as a company is to provide the best platform for building mobile apps, and so XobotOS will not be a focus for us going forward. But it was a fun experiment to run, and as it turns out, a few technologies have come out of the effort..."
So even though an entirelly C# based Android won't become a reality any time soon sadly, (I like C# and .NET/Mono/Monotouch/MonoAndroid much much more than Java and JVM/Dalvik), some pretty interesting tech came out of the experiment. An exciting one is C# bindings for Skia (Chrome internal rendering engine).