A new study covered in Runner's World:

An Austrian study that made worldwide headlines last week suggesting a link between marathon running and skin cancers, possibly including the dangerous malignant melanoma (MM), turns out to be somewhat less worrisome than many news reports indicated. Of course, sun exposure is a known cause of skin cancers, and runners do go out in the sun (in skimpy clothing), so they should be careful. But the research study uncovered no melanomas among either the 210 marathoners or the 210 control group members, and never attempted to measure skin cancer incidence. "On clinical examination, no skin lesions suggestive of MM were diagnosed in either group," wrote the authors, a group of running aficionadoes and marathoners/triathletes from the University of Graz in Graz, Austria.

However, the marathoners clearly had more liver spots and atypical moles, and received more referrals, than the control subjects. Of particular interest to runners, all three findings were associated with increased levels of training. In other words, those who trained the most (over 44 miles a week) had more liver spots, atypical moles, and referrals than those who trained less. Presumably, the more you train, the more time you spend in the sun. Here's the data on miles/week of training and incidence of atypical moles, liver spots, and "referrals."