All was going smoothly creating the infrastructure for our next solution using the CompositeWPF/Prism package until I added a Silverlight Toolkit Treeview control (all of the standard controls worked just fine).  After adding the Treeview control to the ReportView.xaml file I got the following runtime error:

It was the following link that gave light to the problem (FYI, the designer was happy):
http://silverlight.net/forums/p/12448/48559.aspx#48559 

Which suggested there are issues with inherited controls - troubleshooting this concept I added the Treeview control to my Shell.xaml (ZLCAdmin.Silverlight project) and surprisingly my problem went away; I removed my control from the shell and the solution continued to work!

In the case of the CompositeWPF/Prism infrastructure the fix was easy - simply add a reference to the Microsoft.Windows.Controls.dll in the Main project; in the case above the ZLCAdmin.Silverlight project.