Great Media Center 2005 Article: Writing Background Add-ins
In case you haven't stumbled accross it yet, Stephen Toub wrote a great article on creating background add-ins with Media Center. He gives a good hello world example (and walks you through the plumbing). After that he demonstrates a “position changer” add-in that shows all sort of cool functionality, including the interception of messages from the remote control and how to advance to a certain location when you are playing back video, DVD, fm radio, etc! It's quite cool and definitely one for the tool box.
Link here