I had always assumed Android was limited to Java and iOS (iPhone/iPad) was limited to Objective-C, but you can now use Mono for Android and MonoTouch for iOS to write your apps in C#.
I didn't know this was possible until I read the What's New for Liquid XML 2013 where it mentioned support had been added for XML Data Binding with Mono from a company called Xamarin.
Xamarin got the rights to develop Mono extensions from Novell in 2011, I downloaded the free trial and it is a really great product. It integrates within Microsoft Visual Studio, and has good examples to get started. Within a few minutes I had an app running within the android emulator that is provide in the install. Note: The install is huge (940MB) but includes all of the third party products such as Android SDK.
I think this is a big step forward for developing on these platforms as cross platform support is now made easier...hopefully I can now make a fortune writing the next app to go viral, hmm I wonder if birds and piggies would work?!