Last October the NCBI launched the pilot phase of a program called PubMed Commons, designed to allow users to comment on published abstracts on the PubMed website.
PubMed Commons enables authors to share opinions and information about scientific publications in PubMed. All authors of publications in PubMed are eligible to become members. Members play a pivotal role in ensuring that PubMed Commons remains a forum for open constructive criticism and discussion of scientific issues. They can comment on any publication in PubMed, rate the helpfulness of comments, and invite other eligible authors to join.