At OpenStreetMap, the wikipedia tag is widely used but wikidata less so. A number of us OpenStreetMedians are pushing for wikidata to be used more and eventually supplant wikipedia, since its entity IDs are language-neutral and potentially more stable than Wikipedia article titles. For wikidata to be used more, it needs to be very easy to use in popular OSM editors, particularly iD.
https://github.com/openstreetmap/iD/issues/2680 is the general “support Wikidata” ticket for iD.
https://github.com/openstreetmap/iD/pull/2732 is a pull request to add basic support that rides along with the existing Wikipedia support.
I’m working on this during the Wikimania 2015 hackathon.