ExEn is a brand new tool for developers that's designed to bridge the gap between XNA, Silverlight and iOS (and eventually Android). XNA stands for "Xbox New Architecture," and is the general API used by developers to create games for Microsoft's Xbox console, and ExEn purports to take those games, and bring them over to Microsoft's Silverlight platform as well as Apple's iOS. It's still a work in progress, so some features are missing (the biggest of these being, of course, that Android compatibility), but it's completely free and open source, so any developers who have XNA code that they want to try and port are welcome to go grab it and give it a shot.
If it works (I'm not a developer, so I haven't actually used it), this might put an interesting wrinkle in the developer ecosystem. I had a nice chat with the creator of I MAED A GAM3 W1TH Z0MBIES 1N IT!!!1 (yes, that's really the name) a few weeks ago -- that strangely-named dual stick shooter is perhaps the most famous Xbox Live Indie game title, and he recently ported his title over to Windows Phone 7 with the blessing (if not the full support) of Microsoft. But I wonder what his options would have looked like if a tool like this could have allowed him to easily and quickly bring the game over to iOS.
At any rate, we'll see if any big XNA developers try to take advantage of a system like this. If it works, hopefully we'll see great XNA games, like Cthulu Saves the World, appear on the iPad and iPhone.