Everybody felt it must be true, but there were no hard data on it. That is, until now. The new issue of PloS - Biology just published a study that demonstrates that papers published in Open Access journals get cited more than papers hidden behind the subscription walls (or worse - not available online at all).
Read the editorial introduction: Open Access Increases Citation Rate
Read the study: Citation Advantage of Open Access Articles
And you may also want to read another interesting, related, long study they cite: NEW JOURNAL PUBLISHING MODELS: AN INTERNATIONAL SURVEY OF SENIOR RESEARCHERS (pdf)