I was pointed towards MonoGame 3.0 this week by a colleague (thanks Paul!), and so this morning I thought I would give it a go. I found it very tricky to get it up and running for iOS development (there are installers for the other platforms like Windows, Linux, but not for iOS/Mac). After a lot of trial and error I finally got a project up and running with all the correct references in MonoDevelop.
From there it was clear sailing, as so far as I have found, XNA 4.0 is fully ported over. I very quickly had a little spaceship asset loaded, a GameObject class created, and movement implemented to move the ship wherever the screen is touched.
This should make it easy to develop games for a variety of platforms with almost the exact same codebase.