I was starting to write a 3 part series on the new AddIn Framework in .NET 3.5. However I came across some resources which makes me end this series and rather point to those posts and articles since they have already described it so nicely. Here are the articles and blog posts
MSDN Magazine : CLR Inside Out : .NET Application Extensibility by Jack Gudenkauf and Jesse Kaplan
This MSDN article introduces the concept of System.Addin namespace concepts and how it is structured. This actually provides
MSDN Magazine : CLR Inside Out : .NET Application Extensibility Part 2 by Jack Gudenkauf and Jesse Kaplan
This is the second article of the extensibility series that describes the pipeline architecture that was used, each of the components and how it is implemented. A must read anyone who wants to implement the addin model.
How To: Build an Add-In using System.AddIn by Guy Burstein
This is the practical hands on example with code examples and many screenshots. A tutorial cannot get any better than this. A must visit to the addin implementer. Make sure you visit this article, this is written by a tech evangellist at Microsoft.
This is the .NET based blog of the people who made the framework and contains many inside information you may not find in the documentation. It is a nice place to visit every once in a while.
Everything you need to know to get started with System.AddIn by James Green
This is another good start point for addin examples with visual diagrams.