Devon Greyson et al. have self-archived "Open Access Archiving and Article Citations within Health Services and Policy Research" in E-LIS.
Here's an excerpt:
Promoting uptake of research findings is an objective common to those who fund, produce and publish health services and policy research. Open access (OA) is one method being employed to maximize impact. OA articles are online, free to access and use. This paper contributes to growing body of research exploring the “OA advantage” by employing an article-level analysis comparing citation rates for articles drawn from the same, purposively selected journals. We used a two-stage analytic approach designed to test whether OA is associated with (1) likelihood that an article is cited at all and (2) total number citations that an article receives, conditional on being cited at least once. Adjusting for potential confounders: number of authors, time since publication, journal, and article subject, we found that OA archived articles were 60% more likely to be cited at least once, and, once cited, were cited 29% more than non-OA articles.