As of May 25, 2008:
NIH applications, proposals and progress reports must include the PubMed Central reference number (PMCID) when citing an article that falls under the revised NIH Public Access Policy. The PMCID will need to be noted on documents such as the Literature Cited section and the Publication List that are included as part of applications, proposals and progress reports.

All manuscripts added to PubMed Central (PMC) are assigned a unique identifier called a PubMed Central reference number (PMCID). The PMCID is a number that is assigned to the submitted manuscript after the final quality assurance check by PMC. Until the PMCID is assigned, a NIH Manuscript Submission system reference number (NIHMS ID) is used to track the manuscript. The NIHMS ID should be cited if there is no PMCID reference number assigned yet.

If neither the PMCID or the NIHMS ID reference numbers are available, cite as “PMCID pending.”

For more information see: How to Locate and Cite the PMCID Reference Number.