Entlib Extensibility Labs final release includes the Custom Application Block tutorial
Just a quick note: Extensibility Hands-on Labs for Enterprise Library have been updated to include guidance and exercises on building custom application blocks – both runtime and design-time pieces. The full set of labs is on MSDN now. A friendly reminder, if you build an extension to EntLib that others may potentially benefit from, consider releasing it on EntLibContrib.
See also my earlier post on Enterprise Library Extensibility.