Thanks for the reply,
The issue with the sample you provided, and I may not have been clear, is that it's very specific to a particular viewmodel property and collection. I need to push this down to a framework level to intercept when any of our ComboBoxes try to bind to a selected value that is not in their ItemsSource. Does that make sense? So I'm looking for some hook in the control class itself to detect this case. I believe that I've tried overriding the actual SelectedValue dependency property but I don't remember that working. However, something like that would be an example of handling the generic case when the selected value does not exist in the ItemsSource.
Thanks!
Adam.g