I'm not sure how pornography could cause rape, for a few reasons:
1) Let's think about it scientifically - if you were trying to prove that "A" happening results in "B" (let's say for example, smoking causes cancer), you would need to produce numerous case studies where the occurrence of A, isolated from other factors, almost always leads to B. If, for example, you surveyed 1000 smokers with reasonably consistent other habits, and say 700 of them developed cancer, then yes, there would be a correlation. 500, sure. 300, maybe, 100, I guess so - until you got down to a level where you couldn't say there was a positive correlation anymore.
Think about the millions of people who look at pornography every day. Take the circulation numbers of certain magazines, add that to the number of video tapes/DVD's rented and sold, the traffic on adult newsgroups and websites, etc...and you get a boatload of people. And how many of them are rapists? I, personally, haven't seen one case where a person was convicted as a rapist, and viewing of "normal" pornography was cited as the only cause -- true, there may have been porn consumers who were rapists, but usually they had a myriad of other problems as well. In that case, no correlation, and the hypothesis is false.
2) In every study I've seen, as well as many reports I've read and heard from rapist survivor groups, I've always been told that rape is a crime committed to rob the victim of power - not to solely (or in most cases, even primarily) satisfy a sexual urge. If the motive behind the crime in question is not a sexual one, why would viewing sexual material contribute to an increase in its frequency? Realize, too, that I am not talking here about child porn, or porn in which everyone involved was not a willing participant. As that skirts through the boundaries of sexuality, and veers right into the realm of power over individuals, I wouldn't be surprised to find that those types of films did make the rapist more susceptible to act. But mainstream porn? No. The numbers are just too far skewed in favor for me to believe that.