Note on the below —
“NetNewsWire”: already produces OPML that tags feeds as type “rss”. Reverse compatibility nearly requires others to follow suit; I’m sure NetNewsWire is not the only application that does this.
However, the more I think about it, the more I think it’s a good idea to distinguish between multiple feed types. I would like to know if a feed is Atom or RSS before I grab it. Saves time, saves CPU on both ends of the transaction, saves network, and so forth.
So yes: type should accurately identify the type of feed, whether that be RSS, Atom, or something else entirely. NNTP, say.