via Pedro Beltrao: https://twitter.com/pedrobeltrao/status/392668221993529345
PubMed Commons is a system that enables researchers to share their opinions about scientific publications. Researchers can comment on any publication indexed by PubMed, and read the comments of others. PubMed Commons is a forum for open and constructive criticism and discussion of scientific issues. It will thrive with high quality interchange from the scientific community. PubMed Commons is currently in a closed pilot testing phase, which means that only invited participants can add and view comments in PubMed.
And there is a new field for Entrez : via https://twitter.com/andreaketchum/status/393052424224534528