Just found a case study showing how an iPhone application was ported to Windows Mobile. The document describes the process of porting Amlitude (an iPhone app for capturing and amplifying of various sounds) to Windows Mobile 6.5. The device used for testing was a HTC Touch and the development environment was Visual Studio. The developers could use either C/C++ or C# for development - the choice was C# since many functions were easier to implemwnt with C# calls. The graphics of the application are completely GDI-based (insted of DirectX).

The actual document can be viewed here and it describes the complete steps the developers took to port the app in its full functionality.

Please note, that this article did not focus on deployment - the deployment of an application for the iPhone is not free, while the deployment for WM is (you only need the .NET Compact Framework and a WM-compatible device).

Seems like this could be a step several developers are going to take (if not taking already) - with the rising popularity of various mobile platforms, developers will start creating their mobile applications for several platforms at the same time, since the porting process is not too complicated with the currently available set of frameworks and SDKs.