14 October 2010 Update: See How to Find the PMCID or NIHMS ID for more information on how to find the PMCID or the NIHMS ID.
One stipulation of the revised NIH Public Access Policy is that as of 25 May 2008, PIs will be required to note the PubMed Central reference number (PMCID) on any NIH applications, proposals, and progress reports when citing an article that falls under the policy. For more information: http://publicaccess.nih.gov/FAQ.htm#c6
PIs can locate the PMCID reference numbers by doing a search of their publication in PubMed. The PMCID displays on the abstract page on the lower right side of the page. If the publication is not yet available for viewing in PubMed Central, the PubMed record will also display the date that the publication will be available for viewing.