Am 09.11.2015 um 03:26 schrieb S Page:

I think these other identifiers are all "Wikidata property representing a unique identifier" and there are about 350 of them [2] But surprisingly, I couldn't find an easy way to look up a Wikidata item using these other identifiers.

We discussed some loose plans for implementing this in Currus when Stas was in Berlin a few weeks ago. On Special:Search, you would ask for property:P212:978-2-07-027437-6, and that would find the item with that ISBN.

Stas: do we have a ticket for this somewhere? All I can find are the notes in the etherpad.

Also, is this a temporary thing? Will Wikidata eventually have items for every book published, every musical recording, etc. and become a superset of all those unique identifiers?

It's highly unlikely that wikidata will become a superset of any and all vocuabularies in existance. Better integration of external identifiers is high on our priority list right now. The first step will however be to property expose URIs for them, so we are no longer a dead end in the linked data web.

But since we need to work on Cirrus integration anyway, I expect that we will have search-by-property soonish, too. I certrainly hope so.