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.
And more here … Silverlight 4 demos from my PDC Keynote now available – ScottGu’s Blog
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?
Via What Is The Difference Between an IoC Container and MEF? – Jak Charlton – Insane World – Devlicio.us – Just the Tasty Bits