From http://www.sciencecodex.com/public_access_mandate_made_law:
President Bush has signed into law the Consolidated Appropriations Act of 2007 (H.R. 2764), which includes a provision directing the National Institutes of Health (NIH) to provide the public with open online access to findings from its funded research. This is the first time the U.S. government has mandated public access to research funded by a major agency.
The provision directs the NIH to change its existing Public Access Policy, implemented as a voluntary measure in 2005, so that participation is required for agency-funded investigators. Researchers will now be required to deposit electronic copies of their peer-reviewed manuscripts into the National Library of Medicine’s online archive, PubMed Central. Full texts of the articles will be publicly available and searchable online in PubMed Central no later than 12 months after publication in a journal.