Submission + - PubMed Commons opens up scientific articles to user comments (plos.org)
Recipients of NIH (US) or Wellcome Trust (UK) grants can go to the NCBI website and register. You need a MyNCBI account, but they are available to the general public. If you are not a NIH or Wellcome Trust grant recipient, you are still eligible to participate if you are listed as an author on any publication listed in PubMed, even a letter to the editor. But you will need to be invited by somebody already signed up for participation in PubMed Commons. So, if you have a qualifying publication, you can simply get a colleague with the grant to sign up and then invite you.
However, reports are in that anyone with a PubMed / NCBI account can sign up on the PubMed home page: http://www.ncbi.nlm.nih.gov/pubmedcommons/join/