With MonoTouch, you can now develop and run your .net applications on IPhone and IPod Touch. MonoTouch is a specific version of Mono (http://www.mono-project.com/Main_Page) for these devices. For more information: http://monotouch.net/
Know that you can develop .net application on windows Mobile with the .net Compact framework (http://msdn.microsoft.com/en-us/windowsmobile/bb264329.aspx) and store your application on the Windows Mobile Market Place (http://developer.windowsmobile.com/).