Hi Warren,
I was able to run Agustin's sample with the same system setup as yours, except for one difference: I used the4.1 version of Prism instead of version 4. As the sample is created with Prism 4.1, this could be the cause behind why you needed to re-add the references to thePrism libraries.
As far as I know, Prism 4 targets Silverlight 4 andPrism 4.1 targets Silverlight 5. However, in the description of your system setup you said that you are usingSilverlight 5 with Prism 4 and this could be causing unexpected behaviors in your application.
Please, try again using the Prism 4.1 libraries for Silverlight 5 and let us know if this solves the problem. Alternative you could change the target version of the projects of the sample toSilverlight 4 to use Prism 4.
You can find Prism 4.1's assemblies in the following link:
As a side note, if you wish to register Prism 4.1's assemblies in visual studio, take into account that there is a known bug in the script used to register the assemblies. You can find more information about it an how to solve it in the following work item:
Regards,
Damian Cherubini
http://blogs.southworks.net/dcherubini