Dienekes had a post on this paper in AJPA...plenty of good comments too.
The authors failed to support the hypothesis that as you move away from the equator, sexual dimorphism in skin color should increase. This hypothesis is based on the effect of sexual selection for lighter females and darker males.
In one of his comments, Dienekes raises an interesting question which is whether genes involved in skin pigmentation are sex-linked. Sexual dimorphism in skin color (that is found in many populations) might be sex-linked indirectly through effects of hormones, not the effects of skin color genes, per se, perhaps?
one more interesting thing: Apparently, according to the authors, the vitamin D hypothesis for lighter skin is not so convinving due to "a lack of paleopathological evidence of rickets , and the abundant dietary adaptations of humans living in such areas to acquire the component."
By the way, Jared Diamond (The Third Chimp) and Henry Harpending, I believe, have discussed the importance of sexual selection for skin color, since there are areas in the world that are not exposed to very much sun (new Guinea , for example), where skin color is dark. There are other examples that escape me right now.