I have to be careful with this blog, as it is labeled Android Development, however, I have made an application that works for both Android and IOS. Now, keep in mind, you won't get all the features in Xamarin from this project in comparison to their Mono Droid / Mono Touch sdk's, but it does provide enough to make real applications. MVC Style.
The Monocross Project
What I have found is, the best way to implement this sweet layer, is to make sure that your web services are truly restful. If you are in .net land, and I assume you are, this means RIA Web Services, or the new .net 4.5 API, which is supported in Azure.. with the October 2012 Azure SDK.
In order to utilize this tool, I'll have to do some extensive reworking on my web service side.
I've already planned on doing it, but it will be delayed a little bit, due to side projects currently ongoing.
I've also looked at Cross platform Mobile Game Development environment. Now, the real winner in this arena is Unity. Hands down. They have the gear, and the right environment for full game development, but that comes at an expensive price tag. A lot of people like doing homebrew in the XNA platform. Currently, Mono Game.. supports 2D games. It's working up for 3D games, but for my purposes.. Mono Game.. being free and all, works well enough to get my feet wet. So I've been playing around there.
The Mono Game Project.