I like user contributed annotations to the SDK and at first I thought RSS was a nice approach. But thinking more about it, it seems RSS is really wrong.
What happens when an old annotation gets pushed out of the RSS feed after a certain number of new annotations were added? It's lost for users that didn't cache it at the time it was available...
What's needed instead is a simple query mechanism, where the annotation service returns a list of notes/comments/annotations when asked about a specific topic.