Is anyone looking at Apache CMIS (Content Management Interoperability Services)? Existing tools such as OAI-PMH and OAI-ORE provide great ways to expose and exchange information about Omeka items, but in some cases/roles, it may be very useful to think of Omeka as more Digital Asset Management System than a repository--the ability to share assets with more traditional CMS's in this fashion might be a significant re-use, re-mix tool.
I think the API is intended to serve this purpose: http://omeka.readthedocs.org/en/latest/Reference/api/for_beginners.html
Well, that provides a way to write a query, but it would still require some custom code. I'll try to explore that, along with the Atom interface--the CMIS book claims that AtomPub is a more popular CMIS interface than their use of Web Services.
Yes, the API is a move toward more interoperability, and the relation between an Omeka site and a full-on DAMS is on our minds, though Omeka is much more about the publishing than the asset management. A CMIS tool would most likely require either a new plugin from the community, or something that converts the API output.