It's interesting, if you made RSS over XMPP instead of HTTP, it would solve a whole lot of problems. Nobody does that though to my knowledge. XMPP, the jabber instant protocol, is much better for that sort of stuff. For one thing there's already a specification for publish/subscribe type interaction. For another thing, even ignoring the pubsub system, the process of creating accounts and adding buddies itself is a publish/subscribe sort of interaction. To publish one needs only send a message to every buddy on the list, and to subscribe one needs only to request authorization to be on the list, and allow that JID to send you messages.
I envision a pseudo-jabber client, that creates and logs onto an account you set up, and when that client receives a RSS entry it displays it or plays it or saves it for you to peruse later, and a second pseudo-jabber client that does the same, but sends new RSS entries to all buddies on its list. Anyone subscribing with their normal text client would just see RSS XML being sent as instant messages, or maybe nothing at all if the namespace changes enough.
The problem with RSS over HTTP is, when you update your feed, there's no way to tell other people it's been updated. You have to tell them to call you at regular intervals, and then when they ask you for the page every hour you say, "No it's not changed. No it's not changed. No, it's not changed" wasting both time and trouble, until finally you say, "It's changed!" Then they download the new entry. Another problem is because RSS feeds are stored on a file with multiple entries in it, clients tend to request the whole file just to get the most recent changes, since they can't tell how much of the file has changed until they download it.
RSS over XMPP would solve that in several ways. First off when you update your feed, it tells other people by sending IMs to them, intercepted and processed by their own clients. No callbacks, polling, or any other time wasting junk. You update, you send. Second off, you don't have to store RSS feeds as a file with multiple entries. Heck you don't have to store it at all. Just generate the newest entry, queue it up to send to each person on your buddy list, and then forget the newest entry since it's guaranteed that jabber servers will hold onto those messages until they're assuredly delivered.