In "A Taxonomy of Articles in PubMed Central," Jim Till examines the open access characteristics of articles deposited in PubMed Central that were published between April 7, 2008 and August 7, 2008.
Here's an excerpt:
Summary: The total number of articles published in the 4-month interval (April 7 to August 7, 2008) and contributed to PMC was 23960. The four subtypes of articles in PMC, and their estimated proportions during this 4-month interval, are: 1) Author manuscripts that are publicly accessible (7346/23960=30.7%); 2) Articles that are embargoed (378/23960=1.6%); 3) Articles that are Libre OA (3635/23960=15.2%); 4) Other articles that are publicly accessible, via Gratis OA (12601/23960=52.5%). These proportions are probably not very different for the subset of NIH-supported articles, if it's assumed that, during this 4-month interval, about 50-60% of the articles contributed to PMC were NIH-supported.
