Keiko Kurata, Tomoko Morioka, Keiko Yokoi, and Mamiko Matsubayash have published the "Remarkable Growth of Open Access in the Biomedical Field: Analysis of PubMed Articles from 2006 to 2010" in PLOS One.
Here's an excerpt:
This study clarifies the trends observed in open access (OA) in the biomedical field between 2006 and 2010, and explores the possible explanations for the differences in OA rates revealed in recent surveys. . . .
OA articles in the biomedical field have more than a 50% share. OA has been achieved through OAJs. The reason why the OA rates in our surveys are different from those in recent surveys seems to be the difference in sampling methods and verification procedures.