Insty linked to it a while back but I can't find it, I do however have a rebuttal.
"We believe that the majority of anti-virus products on the market can't keep up with the rate of virus propagation on the Internet," the study stated. "[What] enterprises and consumers spend on anti-virus is not proportional to its effectiveness."
Imperva's study was widely criticized at the time for poor methodology, small sample size and unrealistic testing scenarios. Because of those doubts, TechNewsDaily did not report the study when it was first released."
According to that story Imperva used a small sample size and had other incorrect or misleading methodolgy. Click the above to go read it for yourself,