The NIH Public Access Policy requires that Principal Investigators submit final peer-reviewed journal manuscripts that arise from NIH funds to the digital archive PubMed Central upon acceptance for publication. The policy applies to all manuscripts accepted for publication on or after April 7, 2008.
Compliance requires that these papers are accessible to the public on PubMed Central no later than 12 months after publication. Evidence of compliance must be provided in all NIH applications and reports.
As of July 1, 2013, papers that aren't compliant will cause funding delays for the supporting NIH grants. Detailed instructions on the submission process are available here from the JHU Libraries. The NYU Health Sciences Libraries have put together a great presentation outlining the compliance procedure.