I said specifically skin cancer. Naturally there's a balance between getting vitamin D and avoiding harmful UV. So the best skin tone to have differs in different environments (hence the gradual cline in skin tones over the world).
I don't have details at hand to cite, but I recall encountering some recent news of research that may place in question equating a simple measure of sun exposure to elevated risk for skin cancer. (I didn't look at in depth, and my memory could be somewhat faulty.)
It may not be simple quantity of exposure, but rather severe sunburns that most elevate risk. If one is outside regularly and acclimatized, these severe burns are less likely to occur.
Perhaps someone else will have a more detailed comment on this.
Of course, one's pigmentation will also influence the likelihood of sunburn, although dark pigmentation does not entirely prevent burning.
EDIT: Just read InclinedPlane's comment, above, which mentions the same concern with sunburn.
