AI OnAI Off
I am not an app developer myself but I have been involved in projects including mobile apps and my experience is that it works well to build a JSON REST interface to expose data from the website to the app.
Here's an example of using ASP.NET WebAPI to create such a service: http://www.frederikvig.com/2012/02/using-the-asp-net-web-api-framework-with-episerver/
As I am creating a mobile app / second screen app for one of our website, I wanted to check whether I could get xml-files as output from what we add to the CMS. I know episerver has that functionality, but, I want to know what would be the best way to expose content to external systems