The BizTalk 2004 Server pipeline architecture opens up a wealth of opportunities by allowing you to plug in you own components. Technically, when developing so called "custom pipeline components", only a few basic principles need to be taken into account. As such, nearly any .NET developer should be able to get away with it within a few hours. In order to even further improve this developer experience, this article aims at explaining and introducing a couple of basic principles and techniques.
See here : http://www.microsoft.com/belux/nl/msdn/community/columns/claessens/custompp.mspx