This is actually a UC wide policy set by the office of the President and includes all UC campuses. So the impact of this policy adoption should be fairly large:

"In order to comply with this requirement, UC investigators should enclose this letter signed by William Tucker, Executive Director, Office of Technology Transfer and Research Administration, University of California, Office of the President, with any articles submitted to publishers for possible publication. This letter gives notice to the publishers that if accepted for publication the article will be required by law to be posted on PubMed Central. If this letter is not submitted to the publisher at the time the article is sent in for review, it must be submitted along with any publication agreement signed by a UC investigator."
http://www.ucsd.edu/portal/site/Librarie...

There is also a nice compliance flow-chart at:
http://biomed.ucsd.edu/nih_flowchart.pdf