WCF extensibility guide
The first chapter of the WCF extensibility guide that Jesus and I have been working on during the last few months went live in MSDN this weekend. You can find it here, http://msdn.microsoft.com/en-us/library/ee672186.aspx
The first chapter gives an overview of the channel layer, and some of the extensibility points you can find there to implement custom channels.
Enjoy!!