Private Information Retrieval--Is it still theoretical? Are there any real world products in which private information retrieval techniques are used?(a data hosting server oblivious of which data item was fetched from it?)If not what are the major barriers for its implementation?
Not a real answer, but some hints:

Single DB PIR schemes (ones that don't need several non-colluding DB) have had serious efficiency problems for a long time. See paper 'on the computational practicality of private information retrieval' by Sion and Carbunar arguing that all schemes at that time (2007) were less efficient than downloading the whole DB (most simple PIR technique).

However the same year, Trostle and Parrish published a PIR scheme that does not have this problem. If someone uses PIR he's probably using this one.

We use it in my team but it's research, and I am not aware of use cases in the industry.

EDIT

As mentionned by DrLecter, Trostle and Parrish single-database cPIR has been broken (http://eprint.iacr.org/2015/012).

Good news are that PIR has evolved since then and a French team built a PIR framework to prove that PIR is now practical: XPIRe: Private Information Retrieval for Everyone

The PIR they use does more or less the same thing as Trostle and Parrish, i.e encryption of a zero for cells you don't want, encryption of 1 for cell(s) you want, but they use a stronger underlying homomorphic encryption scheme so that it's not broken. The obtained throughput and latency they obtain is insane (1080p streaming over NetFlix-like DB !!!).