This is very old news, but it's worth repeating.
The highly respected WCF Sensei -- Nicholas Allen -- has an excellent article on the binding stack in WCF, how WCF is extensible, and specifically, where and how you can customize these points. I've been looking for this kind of treatment of WCF customization for some time. I really could have used it when I was struggling with building a custom interop solution for a client, but alas. Regardless, I'm noting it here for posterity and for anyone else out there trying to grasp these concepts.
Anyway, the article starts out at http://blogs.msdn.com/drnick/archive/2007/02/19/channel-development-tour-part-1.aspx. It is a lengthy series of bite-sized articles on various aspects of WCF. Good stuff! Thanks, Nicholas!