Interesting, arxiv:0709.0896, “Open Access does not increase citations for research articles from The Astrophysical Journal” by M. Kurtz and E. Henneken:
We demonstrate conclusively that there is no “Open Access Advantage” for papers from the Astrophysical Journal. The two to one citation advantage enjoyed by papers deposited in the arXiv e-print server is due entirely to the nature and timing of the deposited papers. This may have implications for other disciplines.
It would be interesting to know if the same can be said about much less prestigious journals than ApJ.
I doubt that it has “implications for other disciplines” – every discipline has a unique set of “traditions”. In fact, it would probably be absolutely not true for high energy physics: papers, not submitted to ArXiv prior to/after publication are largerly ignored. Moreover, papers posted on ArXiv but never subitted to a journal could be highly cited…