Could you confirm that your solution is indeed using the dev assemblies? RadControls_for_Silverlight_2009_1_612_dev.zip is a Dev release and no error messages are shown.
Sometimes even if you add referencies to new assemblies the project might still use old assemblies that are located in the project's folder. If you open the properties tab for your assemblies you will see a property called Path that will tell you the exact assembly that is being used. Could you confirm that your project using the new assemblies?
the Telerik team