I really liked these videos. It is probably the most detailed MEF video/intro that is out there.
MEF is not for beginers and it should be noted as an "advanced" topic. Devs who are not familiar with attributes, interfaces, lazy loading patterns, Silverlight app structure and to some extent Visual Studio 2010 are simply going to be scratching their
heads after the first few minutes.
The only feedback I would give is to possibly the source code for each video (as you state, you change the code from video to video in some cases) and if someone created a lab for this that would be great as well.
The next evolution of MEF needs to be some "best practice" guidance from Microsoft on using MEF.