XNA is a set of C#-based tools by Microsoft to facilitate game development. The framework runs on the .NET Framework 2.0 for Windows, and the .NET Compact Framework 2.0 for the Xbox 360.
Independent developer, Andrew Russell, has now ported XNA to run on iOS and Silverlight (an Android version is not far away) - and it's called ExEn. Because it's licensed under MS-PL (Microsoft Public License), developers can use ExEn in their games right away. The first public preview is now available. Those who have donated to the project will get special access to the pre-release program.
While Microsoft would like to see iOS games being ported to Windows Phone, we could now see XNA-based games (on Windows, Xbox 360 and Windows Phone) being ported to other platforms instead. Currently, ExEn handles sprite-based games only.