i would build something you find interesting/useful. after the project is complete you can continue development and maybe it will catch on.
xml is just a format. and a rather verbose format at that. you can also transfer data using json and serialized objects.
the only downside I see to developing a mobile app is you are locked into that device: ipad, iphone, android, windows mobile. if you create a web application with a mobile friendly view you won't be locked into a single device. all the aforementioned devices would be able to access the app.
good luck with your project.
Jason Meckley
Programmer
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