One of the demos that really sticks in my mind from the whirlwind that has been MIX07 was Silverlight Airlines. It was a demo that Scott Guthrie presented in the Day 1 keynote, showing how to integrate the .NET Common Language Runtime with Silverlight. The premise of the demo was to improve on the typical experience of booking a flight, using Silverlight to dynamically show various routes between your starting point and destination.
It was the perfect demo showing the power of integrating Silverlight with CLR technology, and now the demo and the associated code is available to you! Check out Delay's blog post on it at http://blogs.msdn.com/delay/archive/2007/05/01/the-web-just-got-even-better-silverlight-announced-at-mix07.aspx.