Bryan Field-Elliot of PingID makes the connection between something like RSS-Data and the need for true pub/sub style infrastructure for RSS. Like others, he's frustrated by the “pull” nature of subscribing to RSS data feeds.
In a separate email, I pointed out to him that RSS 2.0 already provides functionality (in the <cloud> element) for this, though it's not implemented in many reader/aggregators (Radio UserLand does implement it). Dave called this “Soap meets RSS“, and it's documented in the spec.
Data-oriented syndication applications on RSS need this kind of functionality, and it's already there, though virtually no one implements it. [Jeremy Allaire's Radio]