Google tells me that issues with the Silverlight ComboBox control are widespread and well known. It also tells me that there are few solid solutions out there if you want data binding to work with the Silverlight ComboBox in a simple, reliable manner.
After wasting more time than I care to consider, I believe I have a more complete solution. Though it isn’t perfect either, it does appear to work nicely with CSLA .NET NameValueListBase objects, and inside DataTemplate elements, which is really what I was after.