This post of Manish Dalal outlines a technique for displaying ComboBox control with .net RIA services to handle Foreign Key scenarios.
When ComboBox control was introduced as part of Silverlight 2, I blogged about a technique to use ComboBox control in foreign key scenarios, to workaround the lack of SelectedValue\SelectedValuePath property. I further enhanced the technique to handle cascading selection as outlined in this post. Fast forward to Silverlight 3. ComboBox control still does not have SelectedValue\SelectedValuePath property and the same technique still works. Moreover, if you are using Silverlight 3 with .net RIA services, majority of the code is automatically generated by the framework. In fact, with Silverlight 3 and .net RIA services there is no requirement for SelectedValuePath property!