A few months ago I found this great blog by John Kozell at Microsoft. He showed us how to build custom SharePoint Search extensions that I though was very innovative and included in my SharePoint 2007 Search Customization course. Here is a link to the code that I modified to use the new .NET 3.0 classes.
http://blogs.msdn.com/johnkoz/archive/2009/03/05/creating-security-descriptors-for-moss-2007-managed-protocol-handlers.aspx
Enjoy!
Chris