A recent study published in the Public Library of Science has tested the relationship between Open Access self-archiving of peer-reviewed articles and improved citation impact.
See: Gargouri Y et al. ‘Self-Selected or Mandated, Open Access Increases Citation Impact for Higher Quality Research’ PLoS ONE 5(10)
The correlation between publications that are freely available online and high citation metrics has been established many times before and is unarguable, but some have questioned (in what strikes me as a stretch of reasoning) whether this correlation can be taken to imply causation. (In other words, they argue, “Yeah but, maybe those open access papers are cited more because people would only upload their really good papers to the Web that would be cited a lot anyway!”) Harnad and co. demonstrate pretty conclusively using controlled and tested methods that both voluntarily self-archived papers, and those that are required by funding bodies or institutions to be openly archived, have the same beneficial impact on citation, *and* that this benefit is proportionally even greater for the most high-impact publications.
Like I say, we kind of knew this, but we now have a scientific publication we can cite to demonstrate it even to the skeptics.