Mark J. McCabe and Christopher M. Snyder have self-archived "Identifying the Effect of Open Access on Citations Using a Panel of Science Journals." in SSRN
Here's an excerpt:
An open-access journal allows free online access to its articles, obtaining revenue from fees charged to submitting authors or from institutional support. Using panel data on science journals, we are able to circumvent problems plaguing previous studies of the impact of open access on citations. In contrast to the huge effects found in these previous studies, we find a more modest effect: moving from paid to open access increases cites by 8% on average in our sample. The benefit is concentrated among top-ranked journals. In fact, open access causes a statistically significant reduction in cites to the bottom-ranked journals in our sample, leading us to conjecture that open access may intensify competition among articles for readers' attention, generating losers as well as winners.