David R. Hansen has published "Understanding and Making Use of Academic Authors' Open Access Rights" in the latest issue of the Journal of Librarianship and Scholarly Communication.
Here's an excerpt:
METHODS To understand the scope of author-retained rights (including the right to purchase hybrid or other open access options) at some sample universities, author-rights data through the SHERPA/RoMEO API was combined with individual article citations (from Thomson Reuters' Web of Science) for works published over a one-year period (2011) and authored by individuals affiliated with five major U.S. research universities. RESULTS Authors retain significant rights in the articles that they create. Of the 29,322 unique articles authored over the one year period at the five universities, 28.83 percent could be archived in final PDF form and 87.95 percent could be archived as the post-print version. Nearly 43.47 percent also provided authors the choice of purchasing a hybrid paid open access option. DISCUSSION A significant percentage of current published output could be archived with little or no author intervention. With prior approval through an open access policy or otherwise, article manuscripts or final PDFs can be obtained and archived by library staff, and hybrid paid-OA options could be negotiated and exploited by library administrators.