Xamarin has been providing Mono, a C# framework that can work on Linux for some years, and more recently they have also launched Mono for Android and iOS so that you can write or re-use existing C# application on the most common samrtphone platforms.
But their latest project “XobotOS Research Project” goes much further, as they have entirely re-written Android Dalvik engine based on Java with C#. Considering the million of lines of code in Android, they have found a tool to automatize the Java to C# conversion. This tool is an Eclipse plugin called is Sharpen, and Xamarin has made further improvements to Sharpen which are available on Github.
They already have ported Android 2.x and Android 4.0 to C#. If you wonder why they would do that, look at the “benchmarks” below showing Mono vs. Dalvik implementation of Android for binary tree, jovian and hashtags.