I have a first crack at PopFeed, which I've been using to read my mail via SharpReader.
Originally I couldn't find any details on the message format itself, but then I found the real RFC, which says the message format must conform to this standard. Who knows what the hell I was looking at before. So anyways, I have to go back in and make sure the messages are being properly parsed. Maybe I'll have something to show for it this weekend.
On another note, though the vernacular seems somewhat specific to headlines and web resources, I think RSS is reasonably flexible enough to represent many content entities. And it can really open a lot of possibilities, especially because there are already a number of excellent consumers to that standard (and writing a new one is also relatively easy).
I'm starting to get on the bandwagon.... :)