Julie Arendt, Bettina Peacemaker, and Hillary Miller have published "Same Question, Different World: Replicating an Open Access Research Impact Study" in College & Research Libraries.
Here's an excerpt:
To examine changes in the open access landscape over time, this study partially replicated Kristin Antelman's 2004 study of open access citation advantage. Results indicated open access articles still have a citation advantage. For three of the four disciplines examined, the most common sites hosting freely available articles were independent sites, such as academic social networks or article sharing sites. For the same three disciplines, more than 70% of the open access copies were publishers' PDFs. The major difference from Antelman's is the increase in the number of freely available articles that appear to be in violation of publisher policies.