No, we haven't. (I'm the one who added that TODO item to the page but I was busy with school exams until late last week.)
As of now, we're planning on implementing it using Tracker 0.7 (which has a RDF store with support for SPARQL queries) as an optional alternative to our own vanilla Sqlite backend.
(I don't know if it's of any interest to you, but I also spoke to David Barth from Canonical last week and he expressed an interest in hacking together a CouchDB based backend in the coming weeks.)
For more information, you might be interested in an article that I wrote for the next edition of the GNOME Journal, which will be published next month. The draft can be found online
here.
Regards,
Natan