PLoS Biology has an article with data that supports the hypothesis that open access articles receive more citations than articles hidden behind a toll (summary available here). The author compared open access and non-open access articles in PNAS, controlling for any confounding variable he could think of. The article is open access, of course, so you should check it out.
My own exercise in open access — publishing some original research on this blog — has been put on the back burner as I take care of some other research. You can read the background here. The next step involves a bit of data mining, and I’ll get around to it when I’ve got some spare time.