It is currently possible to share 90% of the code between a Windows Phone and a Windows 8 app. Heck, it is even possible to write the UI of an app once and make it work on both platforms. The binaries are now the same on Windows Phone and Windows 8 (I think you can submit a single one for both apps), and universal apps are already a thing (you can buy it once and get the app on Windows 8 and Windows Phone).

It's a matter of time before people realize they can use Xamarin to port a Windows app to iOS/Android in a matter of days.