As much as I wish this were true, there appears to be a problem with the interpretation of the data from this study and this may lead to an incorrect conclusion. Daniel is on to somthing in the second paragraph of his comment. The frequency of masturbation was correlated with a reduction in the risk of developing prostate cancer. However, just because two factors (e.g. masturbation frequency and cancer outcome) are correlated does not mean that one factor actually influences the other. (Some studies can draw causation from tightly controlled correlations, but that is another discussion)

An alternate hypothesis could be that a large portion of the men share some other characteristic (e.g. lifestyle) or a combination of characteristics (e.g. genetics and lifestyle) in common. It's not necessarily that the conclusion (i.e. regular masturbation is a protective factor) is incorrect, but that there may be more than one way to interpret the data to support an alternate outcome. Essentially, more studies are needed to tease apart all of the things that may influence prostate cancer. It looks promising, but a number of studies have been "debunked" because the results from the first study couldn't be replicated.