<p>The MEF team keep saying it is about plug-in model. So, are we talking about UI plugins? How can we use this stuff in non-UI code? 
I downloaded the code and the examples are all about GUI. Am I guessing it wrong?</p>

Ayende sums up MEF rather nicely here:
http://ayende.com/Blog/archive/2008/09/25/the-managed-extensibility-framework.aspx
Also the herding code lads had a great interview with Glenn Block, one of the MS MEF guys:
http://herdingcode.com/?p=28
http://herdingcode.com/?p=31