How do I 'watch' list box selection change?

In WPF VB.NET 4.0, I have a list box that is populated with a data binding from XML. How can I set a live watch event to the listbox so that the selected item, based on a certain criteria (field is true or false), a button on the form has it's content changed to read something else?

i.e., user selects an item with XML element 'status' is set to 'current', then button would read, Finished. If user selects an item with XML element 'status' is set to 'finished', then button would read, Unfinished.