As I've seen there is ATOM support in the application, if you choose the "Vox" engine.
Currently I'm evaluating this for my blog engine, and I have a couple of suggestions which would be nice to have in MarsEdit.
It seems that the automatic API discovery uses the "engineName" element from the RSD file. So, if my engine is not supported by MarsEdit, it will recognize it as "Other Blogger-compatible" and will try to use XmlRPC even if I return only one ATOM endpoint
<api name=\"Atom\" preferred=\"true\" apiLink=\"http://localhost/a/atom.ashx\" />
So, it would be nice to see that MarsEdit uses the api elements and chooses one which fits.
ATOM does not have any authentication method defined, it depends on implementation. Some of the vendors started to use the X-WSSE header based authentication, which is supported by MarsEdit.
The issues is that this method _requires_ the server to have _plain_text_ passwords for the users, so it's a potential security problem. Besides this, other vendors trust more the Digest or NTLM authentication methods, since these are older and more proven.
It would be nice, if MarsEdit would recognize the WWW-Authenticate header returned by the server, and would be able to use at least the Digest authentication. NTLM is harder, and is usually used on intranets, so I think it's not so important.
And there is another ATOM auth. proposal, however I'm not usre if anyone is using it in production. http://bitworking.org/news/New_AtomAPI_Implementation_Release2