Atom + MIME?
I've mocked up what I think an Atom POST request for Tim Bray's How Fast is This Thing Growing? blog entry would look like using the MIME Multipart/Related Content-type. I chose this particular entry as it has a title, a summary, full content, and two pictures - one displayed inline, and one by reference.
I've posted two versions: a 7 bit safe version using Quoted-Printable and Base64 transfer encodings, and a 24% smaller 8 bit version using 8-bit and binary encodings. Clients would be permitted to transmit any combinations of these. Note: what you are seeing is the actual bits that would be transmitted - HTTP + MIME + XML + Atom + XHTML + PNG, complete with authentication.
Unless somebody identifies a showstopper, I'll submit a proposal.