Here are some of the sessions at MIX08 that highlighted or included Expression Encoder:

http://sessions.visitmix.com/?selectedSearch=T09 Overview of Expression Encoder 2
http://sessions.visitmix.com/?selectedSearch=BCT07 Encoding Video For Microsoft Silverlight Delivery Scenarios
http://sessions.visitmix.com/?selectedSearch=BCT05 Integrating Media Into Silverlight Applications (around 0:41:00)
http://sessions.visitmix.com/?selectedSearch=BT06 Silverlight and Advertising (around 0:40:50)
http://sessions.visitmix.com/?selectedSearch=KYN0801 Day one KeyNote  (around 1:09:50)