Library Journal Academic Newswire for July 24, 2008 reports:
[S]ubmissions to PMC began steadily rising in December 2007, soon after it became clear a mandatory policy would be adopted in 2008. By the first month following passage of the new policy, January 2008, monthly submissions to PMC hit an all-time high of 1255, and have continued to increase significantly every month so far this year. In April 2008, when the policy officially took effect, submissions spiked even more sharply, rising from 1852 total submissions in March, to 2,765 in April and 2,593 in May. The April/May 2008 figures represent well over double the number of submissions for the same months in 2007 (1,198 PMC submissions in April ’07; 948 in May ’07). Although official figures for June have not yet been posted, the NIH’s Dr. David Lipman told the LJ Academic Newswire the submission totals were higher than May.
The NIH statistics are available here. The graph makes a compelling visual.
http://www.nihms.nih.gov/stats/index.html