There are two posts on the RSS team blog about the use of the Simple List Extensions, which have been implemented by IE7 and the Windows RSS Platform.
The first is Simple List Extensions in action, and it shows examples of live sites (like Amazon.com, eBay and Yahoo) using the RSS extensions to add sorting and filtering capabilities to their feeds.
The second is a longer post on Understanding Lists and SLE, which explains why RSS list feeds are useful and how they work (with diagrams!).
Check them out and let us know what you think in the comments.