You can watch his keynote online here (Sarts at the 53:30 mark in the video).
He showed a number of fun demos during the keynote that highlighted some of new features coming in Silverlight 4. You can download the demos here (source code + VS project file included with each sample).
P.S. The Facebook application will also be available for download in the future.
What Is The Difference Between an IoC Container and MEF?
MEF allows you to write your application as a series of much simpler mini applications, and have those brought together to create a much great whole
IoC container = service level components, MEF = Autonomous Components. A MEF "part" will probably use an IoC container
I recommend reading them highly, they will do a much better job than I could at explaining the detail of the decision points, Should I use MEF for my general IoC needs? and Should I use MEF with an IoC container?