Hey all,
I've been reading some of the technical notes on Wikidata, for example http://meta.wikimedia.org/wiki/Wikidata/Notes/Data_model http://meta.wikimedia.org/wiki/User:Nikola_Smolenski/Wikidata#Query_language
Statements like "[data model] similar to RDF, but allows qualified property values" and "should there be a query language that will enable querying of the data?" concern me a great deal regarding the future of the whole Wikidata project.
It seems to me that whoever is making these technical decisions does not fully realize the price of reinventing the bike -- or in this situation, reinventing data models/formats/standards. Having designed and implemented production-grade applications both on RDBMSs, XML, and RDF, I strogly suggest you should base Wikidata on standard RDF.
I know some/most of you are coming from the wiki background which might be hard to get over with, but if Wikidata is to become a free and open knowledge base on the (Semantic) Web, then RDF is the free and open industry standard for that. Whatever little advantage you would get from developing a custom non-standard data model, think how many man-years of standardization and tool development you would loose. Isn't knowledge about standing on the shoulders of giants? RDF has all the specifications, a variety of tools, and DBPedia as a very solid proof-of-concept (which I also think should be better integrated with this project) necessary to build Wikidata. With SPARQL Update, full read/write RDF roundtrip is possible (and works in practice). It also makes the notion of API rather obsolete, since SPARQL Update (and related mechanisms) is the only generic API-method one has to deal with.
To round up -- I think failure to realize the potential of RDF for Wikidata would be a huge waste of resources for this project, Wikipedia, and the general public.
Martynas graphity.org