I've found a strange corner case with the RadComboBox. I am currently using the most recent controls (RadControls for WPF Q1 2012), but cannot comment on whether this issue existed previously.
The issue occurs in the actual drop down list when you click on the combo box. All items in the list state "System.Data.DataRowView". However the actual content seems to be bound properly since if you click on an item, the expected value will actually be selected and properly displayed once the list retracts.
What's most curious about this is that everything works fine using the other themes I've tried (Office_Blue, Office_Black, and Office_Silver), which leads me to believe there is an issue specifically within the Expression_Dark themed template for RadComboBox.
Expression Dark Example
Office Blue Example
As mentioned previously, this only appears to occur when binding to a CollectionViewSource who's source is a DataView. Is there a workaround to get DataViews working?