Mono will be the cross-platform tool for Android and iOS development

The nice folks on the Novell’s Mono team are almost ready with MonoDroid – a C# version of the Android API, and MonoTouch – the C# version of the iOS frameworks has been around for a while now. So, if you are about to start a new iPhone/iPad application and you already know C#, you should definitely check MonoTouch. If later you decide to support Android, it will be easier to port your app from MonoTouchto MonoDroid than to rewrite it from Objective-C to Java.