This work is a confluence of two projects - our hstore and json, which were existed in PostgreSQL for years in somewhat incomplete forms - hstore (10 years old!) implements key-value storage with binary storage and indexing, while json - document storage implemented as a text. I and Teodor Sigaev have started working on nested hstore more than year ago with support of Engine Yard and got working prototype with everything document-oriented storage needed (see our presentations in Ottawa, 2013, Dublin, 2013, http://www.sai.msu.su/~megera/postg
Jsonb has several missing useful features (from nested hstore), but that could be added later as an extension.
I hope all database people understand, what that project mean for PostgreSQL - 99.9% percent of projects don't need MongoDB and could be nicely implemented in relational model. Now, PostgreSQL (AFAIK, the first considerable RDBMS) introduced the real support for semi-structured data, which are common in nowaday world.
For historical reason, we saved our nested hstore (hstore v2.0) in separate repository (http://www.sigaev.ru/git/gitweb.cgi?p=h
I, Teodor and Alexander are working on better jsonb indexing. Hope, we'll have something to show at PGCon in Ottawa this year, stay tuned !
This is a screenshot of my message to Teodor Sigaev, which starts our nested hstore project.