The current Chemistry code base has an initial version of the API/SPI together with some actual implementations around the AtomPub protocol. Already Chemistry can talk to itself (AtomPub client talking to AtomPub server) and store data in-memory (which is very handy for unit tests). Outside of the Apache code base, Nuxeo has also coded a backend to provide access to Nuxeo 5.2 repositories using Chemistry. Generic CMIS AtomPub clients like CMIS Explorer are able to see a Nuxeo repository through Chemistry for instance.
Read more on the Nuxeo blog.