If you are in the Phoenix area, I'll be speaking at the Southeast Valley .NET User Group (in Chandler) on Thursday night (June 28). Details are available here: http://www.sevdnug.org/Home.aspx.
The topic is "IEnumerable, ISaveable, IDontGetIt: Understanding .NET Interfaces". We'll see how we can use interfaces to add "seams" to our code. This makes our code more robust, extensible, and testable. Be sure to stop by if you can.