I've had two seperate conversations about this today so I put up a quick wiki page to talk about RSS Best Practices

Also, work on my S-Expression library for Objective-C has been progressing nicely. So far the biggest difference between Rivest's reference implementation and my own is that his is List-based and mine is Tree-based. A b-tree just made more sense to me in the context I want to use it in (a basic Object store).