It is clear that Mono Touch is part of the solution as it allows C# code to run on the IPhone,
however Mono Touch wraps the IPhone API so does not provide a common API.
Another option would be a jave-script toolkit that work on both and provided on-line data storage.
However what real life experience do people have?
MonoCross is a Model-View-Controller system that sits on top of MonoTouch, MonoDroid and SilverLight. So letting more of the UI logic be in common.