Essentially what this draft does is define a way to upload media and the atom entry about that media (in AtomPub jargon, those are called the Media Resource and the Media Link Entry respectively) in one HTTP POST to an AtomPub server, instead of two as per RFC5023. A number of people have implemented "something like this" on their own stacks already; I can see it becoming a default approach to media posting.
The technique uses MIME multipart, specifically multipart/related and not multipart/form-data, which I suspect people working with browsers will be more familiar with. As a result, I'd be interested in hearing anyone's experience implementing atompub-multipart via a browser/ajax client.