[yome]

I suggest you look into Unity 3D. It's easy to make 2D stuff with it, even if it's primarily a 3D engine.

The basic version is free, so you can use that for development. When your game is ready to ship, you can upgrade your license to an iOS one and then convert and ship your game with it.

If you want to stick with XNA, there's an interesting project to port XNA to iOS platforms (http://rockethub.com/projects/752-exen-xna-for-iphone-androi...), and there's XNATouch too. I haven't looked into those yet, but if you complete your game on XNA, by the time you'll be done there might be a viable solution to easily port your game to iOS.