It seems the lack of Vitamin D is connected to more and more health problems. Here's one recent study that looks at Vitamin D and the incidence of leukemia. During the sunny season, leukemia rates decreased by 58% per unit of solar radiation. This is quite a signficant amount!
Also, in this newer study, childhood cancer was connected with lower rates of bone turnover, suggesting a potential role for Vitamin D again. Even if Vitamin D only plays a contributing role in childhood cancers, Vitamin D is a cheap, safe supplement that could be used as preventive medicine.