Did you ever try to give a button one or two rounded corners (not the whole object, just some of the corners)?

Turns out that for pre-3.1 it will round the wrong corners (http://code.google.com/p/android/issues/detail?id=9161), meaning you need "corrected" drawables for API level 12+

That's one example of things that definitely made the whole XML experience a little trippy for us.