First, I wouldn't recommend calcium supplements.

Second, unless you have really, really good evidence, you just can't argue with a randomized placebo controlled trial finding that 1100IU of supplemental vitamin D (and calcium) prevented 77% of cancers, after excluding the first year (which is standard in cancer prevention trials). Here's the study from 2007 with the findings I described: http://www.ajcn.org/content/85/6/1586.full.

Especially giving the mountain of consistent epidemiological evidence, such a strong finding can only be refuted with another larger placebo controlled trial of superior design. They are conducting one now and we will have results in a few years.

That said, I think that some people take it too far, exceeding levels that are physiologically possible. Kurt Harris, for example, incorrectly states that people can obtain levels up 100-120 ng/ml from the sun when the number is more like 50 ng/ml. That's dangerous mis-reporting on his part and naive acceptance of fanciful claims on the pat of his readers. (In fact, since he's a pretty smart guy, I'd guess he probably just made a units error and meant nmol/L).

Also, it's hard to gauge vit D supplementation in isolation. Vit K, vit A, calcium, and magnesium status all play a part, along with numerous other nutrients.

In the face of the uncertainty, this is as area where it really does make sense to let evolution/paleo theory be your guide. Maintaining levels at 40-50 ng/ml is quite natural, in the tropics. We can assume that vit K and vit A levels would also be somewhat high, since both originally derive from green and colored vegetables, respectively, and indirectly from the animals that eat them. Thus, in the northern summer or in the tropics, moderately high vit D levels would likely have gone hand in hand with decent intakes of vit K and vit A, as well as magnesium. I don't think calcium supplementation makes much sense in this context.