The Law

The NIH Public Access Policy implements Division G, Title II, Section 218 of PL 110-161 (Consolidated Appropriations Act, 2008) states:
                            http://grants.nih.gov/grants/guide/notice-files/NOT-OD-08-033.html

The Director of the National Institutes of Health shall require that all investigators funded by the NIH submit or have submitted for them to the National Library of Medicine’s PubMed Central an electronic version of their final, peer-reviewed manuscripts upon acceptance for publication, to be made publicly available no later than 12 months after the official date of publication: Provided, that the NIH shall implement the public access policy in a manner consistent with copyright law.

The Omnibus Appropriations Act of 2009 makes the NIH Public Access Policy permanent.

                       http://grants.nih.gov/grants/guide/notice-files/NOT-OD-09-071.html

The Date
The policy applies to all peer reviewed NIH sponsored research accepted for publication on or after April 7, 2008.

Does it apply to you?

Yes, if your peer-reviewed article meets any of the following criteria:

• Directly funded by an NIH grant or cooperative agreement active in Fiscal Year 2008   
      (October 1, 2007- September 30, 2008) or beyond;
• Directly funded by a contract signed on or after April 7, 2008;
• Directly funded by the NIH Intramural Program
• If NIH pays your salary.

A reminder that NIH funding includes not only grants, but cooperative agreements, and contacts. If the author acknowledges NIH support in the article it should more than likely comply with the policy.

Charts from Becker Medical Library: "When do NIH funded authors need to comply?"
and NIH Compliance Flowchart.

Who is Responsible?
The Principal Investigator or Grantee is responsible, whether or not he or she is an author on a particular manuscript, to make certain personnel, even subcontractors, adhere to the NIH Public Access Policy.