They have a short report on this in the July 21, 2009 Wired Campus
Not all authors or publishers will choose the dual path of free online and paid print publishing. But for those who have a sense of moral obligation to disseminate their work as broadly as possible, there is good news. The common-sense notion that providing free digital copies of a work decreases its sales is incorrect in some circumstances. Additional research is needed to understand these circumstances in sufficient, actionable detail.John Hilton's blog Wide Open is interesting reading.