Laurent Bugnion has found two issues with Windows Phone 7 ApplicationBar buttons so he wanted to post the workaround he used to make this work.
The issue here is that pressing a button on the ApplicationBar does not remove the focus from the TextBox where the user is currently typing. If the button is a Save button, this is super annoying: The Binding does not get updated on the data object, the object is saved anyway with the old state, and noone understands what just happened.
In order to solve this, you can make sure that the Binding is updated explicitly when the button is pressed[...].