One of the cool demos I saw around IE7 was how to extend the RSS platform so that you could include additional information, or provide additional ways of listing and sorting through data that you’d have in the RSS feed. 

Sean Lyndersay on the RSS team pointed me to these links that I found really informative.

Generic RSS info: http://msdn.microsoft.com/xml/rss/

Simple List Extension Example: http://msdn.microsoft.com/xml/rss/sle/