Setting listbox selecteditem in dataform in silverlight not working

Question

I've got a child window that I popup to create events. I'm using the DataForm control and one of the fields is a selection from a listbox of event types. I can see that when the window closes, the event type is set properly on the event. If I pop the window back up and set the event to the one just created, all the fields populate fine except for the listbox control that contains the event type. I've got two-way binding set on the listbox, so I'm trying to figure out what I'm missing to select the appropriate item in the listbox on an edit. Here is the XAML fro the listbox:

<ListBox x:Name="lstEventTypes" 
         ItemsSource="{Binding Data, Source={StaticResource eventTypesSource}}" 
         SelectedItem="{Binding EventType, Mode=TwoWay,NotifyOnValidationError=True, ValidatesOnExceptions=True}"
         DisplayMemberPath="Name" />