After discussing various ways to do dynamic XAP discovery in MEF with Jeremy Likness, Jeff Prosise wrote a sample to demonstrate a technique based on passing InitParams to the Silverlight control.
I've spent copious amounts of time lately digging into Silverlight 4's new Managed Extensibility Framework (MEF) and seem to find new uses for it every day. I love the fact that you can download a remote XAP file and import all of its exported parts with basically two lines of code.