WPF Datagrid: SelectionChanged event isn't raised when SelectionUnit="Cell"

I'm using the WPF toolkit datagrid. I have it set to SelectionUnit="Cell" and SelectionMode="Extended".

The SelectionChanged event is never raised!

It works fine when the SelectionUnit is set to FullRow.

Am I Missing something?

BTW, the reason I need it is since I'm trying to create an Attached Property to help me bind the SelectedCells to my ViewModel.