Malte, IIRC notifications was coded to consume service contracts, under the assumption that email would just be one of many possible notification types. So adding new types should be a small delta rather than a rewrite. Ernie's post suggests that this is so.
What an interesting idea to have "RSS Channel" be a possible notification type! Assuming it doesn't require unreasonable storage or performance hits, this would enable a totally personalized site update stream. The user could select "My Channels" notification type, and add the notification to a set of user-editable channels each of which generates an RSS feed.
Tempting...
