Almost every example of a Silverlight ListBox that I have found merely shows how to get data bound to it, not what to do once you get it there.  What good is a ListBox that you can’t get a reference back from?

The quickest practical way is to hold your original datasource and use the index of the selected item as you key back into your data.  It isn’t pretty and you wouldn’t want to take this approach with large amounts of data, but then again a mobile device isn’t meant to display large amounts of data to begin with.  Here is the sample of the code to retrieve an object from a List.

if (listBox.SelectedIndex > -1)
var tempObject = _myObjectList[listBox.SelectedIndex];
// Do something important like delete the object
// or navigate to another page using the object data

Of course there are other options, especially if this is a navigation scenario.  You could use a ListBoxItem template that would contain a HyperlinkButton and bind the identifier for your object into the URI query string.  I am still looking for other more elegant solutions but these options can help get your application moving.