Last year the Xamarin developers decided that Google's Dalvik virtual machine for Java on Android is quite young and not as performant and tuned as Mono, so they wanted to try and translate Android's source code to C#. In the past few months they resumed this work on having a C# version of Android and have announced this Java-free version to the world today.
Xamarin is calling the result of this Java-to-C# conversion work the XobotOS. XobotOS is considered a research project that has much of the code now ported over to C# without involving Java. Xamarin developers used a cool called Sharpen to translate much of the code (one million plus lines) from Java to C# and ending with Android 4.0 Ice Cream Sandwich.
Initial XobotOS benchmarks showing this C# version to be much faster than the Java-based Android. Xamarin has published this new code to GitHub. However, they will not be maintaining this mobile operating system going forward for any production use. "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."
The Xamarin team will at least be integrating some concepts learned from this XobotOS work into their future Mono-based products: direct graphics access to Skia will be available rather than involving Java to access this Google graphics library, there's a new version of Sharpen for Java to C# tooling, and Xamarin now has the tools to replace chunks of Java code with C# code. "Our plan is to take elements of the research project and integrate those into our products."
Additional details are available from the Xamarin blog.