Dave Johnson of Roller fame has a great post outlining how the Atom Publishing Protocol will work.
I've commented before that I'd like to support blog clients in Leonardo and felt that a REST style made this much more straightforward.
Fortunately, Atom Publishing Protocol (APP), shares a very similar model to Leonardo so it looks like adding APP support to Leonardo will be fairly easy.
Also, the next version of Leonardo (discounting any bug releases) will switch to supporting the final version of the Atom feed format (assuming it's done in time, which it should be).