Jim Earley has a very thoughtful post on metadata interoperability: http://jims-thoughtspot.blogspot.com/2008/04/metadata-interoperability.html
As part of DocBook v5, we added the ability to include content from other namespaces in the <info> block to support adding Dublin Core directly in your content. The <info> element's purpose is to house metadata that is not intended for display, so it's a really good fit.
Jim's argument, is that the various standards out there (DocBook, DITA, ODF at a minimum) should move to Dublin Core for metadata, and stop re-inventing the wheel. Dublin Core is an internationally accepted standard for metadata, so why not use it directly?!
I whole-heartedly agree. This approach would add more compatibility between standards, and maybe even facilitate better search! Along with that, Dublin Core is extensible, so it shouldn't be too difficult to add additional metadata fields if you need to.