The first public preview version of the XNA port for iOS and Silverlight, called ExEn, appeared after Andrew Russell’s authorship; (
on his Facebook page it says that he is an independent game developer from Australia ), which can be
downloaded (direct link) in his blog.
Officially, the port is not supported by Microsoft and licensed under MS-PL (Microsoft's open source licensing). For iOS, instead of the “native” .NET Framework, its MonoTouch port is used. The developer claims that ExEn meets the requirements of Apple, which are presented for iOS applications, so the games developed can be approved for distribution via the AppStore.
The ExEn version for Android is currently in development.
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Description of XNA in the wiki.