Library Journal Academic Newswire reports that article deposits in PubMed Central have dramatically increased (e.g., 2,593 deposits in May 2008 vs. 948 in May 2007) as a result the NIH mandate, which requires "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."
Read more about it at "PubMed Central Submissions Jump Sharply Under New NIH Policy."