You might want to look into the MonoCross project which is designed to help you reuse C# code with multiple presentation layers:
http://code.google.com/p/monocross/
The authors of MonoCross (ITR Mobility) have created multiple mobile cross platform solutions for a variety of customers and have written two books on the subject one is "iPad in the Enterprise" (http://amzn.to/zAhQK6) and the upcoming "Cross-Platform Mobile Development with C#" (http://amzn.to/wM6RsF).
In the meantime, you can watch Scott Olson's presentation that he did at Monospace 2011 that describes how to use MonoCross to target multiple mobile and desktop platforms at once reusing the business logic:
http://www.infoq.com/presentations/The-Rise-of-Mono-in-the-Enterprise
They enforce a strict MVC split in their code:
(source: tirania.org)
The biggest benefit of designing an application with this model is that you can run the same application with native user interfaces on each platform. You get native iOS, Android, Windows UI and they even have an ASP.NET front-end that allows you to publish Web versions of the same business logic.
This technology was used successfully by the Medtronic sample app that is showcased by Apple in their iPad business:
http://www.apple.com/ipad/business/profiles/medtronic/
I suggest you also check out Scott's blog where he posts regularly about his experiences with cross-platform architecture.
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