You can't treat Xamarin as one... e.g. you can't write a simple calculator UI in a cross-platform way - You would have to code the UI part separately for iOS and Android.
At least, that's my understanding and OP (Miguel) confirms it.
And I guess your mileage won't vary in this aspect ;)
Although, Xamarin.Mobile is a step in the right direction, as commented here.
>You can't treat Xamarin as one... e.g. you can't write a simple calculator UI in a cross-platform way - You would have to code the UI part separately for iOS and Android.
Yes, but they do offer a "base" abstraction layer besides the C# libraries, that includes stuff like GPS access, accelerometers, etc.
As for the UI part, one could use a Webkit View as the view, and link the various heavy actions to C# code. For something like a common denominator UI, for a simple calculator or some form based stuff, it would be perfectly fine.
But it also supports the full experience for each platform.