All we need is RFC 4287, plus maybe the service description doc in Atompub.
Atompub tells you how to identify URLs where your application can POST documents of certain types. That's not a whole lot better than some so-called "RESTful" API that tells you how to construct a URL. At design time, I'm learning more about your application than I need to know.
My proposal: Once you've located the URL of a what they call an "edit" link, you should be able to GET a form from that link, describing what you can POST there. Since we already have well defined semantics for the XML elements of Atom entries, an edit URL could return an XForm model:
1. This pattern --- getting a form, filling it out, and submitting it -- differentiates the web, REST style from RPC. In RPC, the programmer learns an API at design time; on the web, clients discover the "API" at run time.
2. We sidestep lots of arguments on the APP mailing list that are driven by different ideas about expected behavior; here, the server tells you its behavior. This server could not honor an atom:id should you submit one. So, it doesn't ask you for one.
3. Extensions understood by the server? Don't worry: if the server doesn't ask you for it, it won't honor it. Example: Atom Threading Extension. A very nice server could offer a catchall
4. We can evolve the Atom syntax without argument. Old servers will never request newly defined elements so will never have to deal with them. I'm guessing this is where Mark is going with reference to semweb.