Hi,

I created a small sample containing a Popup Overlay that covers the whole Shell. In this sample, the View in the Main Region sends a message thorugh the EventAggregator and the Shell shows it in the popup. You can find the sample in my Skydrive account:

• Interaction Popup Overlay with Modules in PRISM
  

This sample was created using Interaction Requests from the MVVM Training In-The-Box:

• MVVM Training
  

Hope this helps,

Federico Martinez
http://blogs.southworks.net/fmartinez