Adam Kalsey: Feed readers need to do the same thing. When I install a desktop reader, the reader should (perhaps optionally) find all the browsers installed on the system and configure them to open files with a content type of application/rss+xml in the reader. The reader then does whatever with it, perhaps showing it to the user and allowing them to subscribe.
Web based readers would need some sort of small install that would redirect that request to them, just as web based mail clients like Gmail need a small program to get mailto: links to open the web mail composition widget.
Of course this would also require that everyone serve RSS as the same content type or for the readers to handle multiple content types. Unfortunately the RSS spec doesn’t specify which content type should be used, so people have made up their own, often different, content types.
Randy: Adam Kalsey is another believer in the Universal Subscription Mechanism. Phil, that always thinking guy, makes a case in Adam's comments for application/feedlist+xml. Something to watch and another great idea.
Randy