Steve Hitchcock of the Open Citation Project has updated his excellent resource, "The effect of open access and downloads ('hits') on citation impact: a bibliography of studies."
http://opcit.eprints.org/oacitation-biblio.html
Here is Hitchcock's introduction:
Despite significant growth in the number of research papers available through open access, principally through author self-archiving in institutional archives, it is estimated that only c. 20% of the number of papers published annually are open access. It is up to the authors of papers to change this. Why might open access be of benefit to authors? One universally important factor for all authors is impact, typically measured by the number of times a paper is cited (some older studies have estimated monetary returns to authors from article publication via the role citations play in determining salaries). Recent studies have begun to show that open access increases impact. More studies and more substantial investigations are needed to confirm the effect, although a simple example demonstrates the effect.Posted by stemp003 at August 21, 2009 5:44 PMThis chronological bibliography is intended to describe progress in reporting these studies; it also lists the Web tools available to measure impact. It is a focused bibliography, on the relationship between impact and access. It does not attempt to cover citation impact, or other related topics such as open access, more generally, although some key papers in these areas are listed as jump-off points for wider study.