Proof of stake was an idea put forward by Gavin to Satoshi in order to solve the issue of people creating scripts and endlessly sending transactions to themselves at zero cost in fees or something along those lines.

Bitcoin has something built in that still does something along the lines of proof of stake. I'll have to hunt around for the reference.

Anyway.....there was a paper that said PoS coins needed to use a centralized reference node:  

http://cointelegraph.com/news/113791/neucoin-whitepaper-reveals-first-mathematically-watertight-proof-of-stake-currency