ExEn is a high-performance implementation of a subset of the XNA API that runs on Silverlight, iOS and Android. (http://exen.codeplex.com/)
From ExEn codeplex site "ON THE FUTURE OF EXEN: I no longer have time to properly maintain and support ExEn (see this blog post for details). On the one hand: ExEn is still an excellent choice for iOS and Android development, due to its stability and performance. On the other hand: I'm not adding new features or fixing bugs (which occasionally crop up when Apple changes something)."
My game for iOs, A comic of zombies (http://itunes.apple.com/us/app/a-comic-of-zombies/id530916013?mt=8), use ExEn. You can visit its website http://acoz.mylittlebets.com/ if you want to download Pc Version or get the lastest news.
Well, the thing is that today I upload a patch for iAd suppot. This and other patchs can be downloaded from http://exen.codeplex.com/SourceControl/list/patches