Tim Bray has a nice summary of how the Atom Publishing Protocol works and a note on the current status.
On the weekend I worked more on my Python implementation of said protocol, Demokritos. I'm close to an initial release which won't be usable as a real Atom server yet but should be good enough for interop testing.
The small hurdle I've just encountered is that (at least in the 06 spec), Atom entries sent from clients to a server need not be fully valid entries. They can omit information that will be subsequently provided by the server itself. The mistake I've make is that my code for parsing Atom entry xml and building an object model is strict about the entry xml being valid. I'll need to relax that for incoming entry POSTs.