This is definitely a situation I would handle via standard databinding. If you want to do this by convention, you will need to add a convention for the AutoCompleteBox. To do this you will need to add some custom code to your bootstrapper's configure
method. This is what the configuration looks like for a TextBox:
ConventionManager.AddElementConvention<TextBox>(TextBox.TextProperty, "Text", "TextChanged");
You can just change the parameters to be appropriate for AutoCompleteBox. The first parameter is the conventional property to databind, the second parameter is the conventional property to use for action parameters (not applicable to WP7 yet)
and the third parameter is the conventional event to wire actions to.