Update: If you can't find the source code in the article to download, you can also get it from here.
It took a looong time and some pretty heavy paperwork, but my first ever MSDN article is up and about on the ASP.Net dev center:
Creating a plug-in framework (the link on the asp.net site)
And this is the direct link into the article on the MSDN library:
Thanks, Kent :)