A trial published in the New England Journal of Medicine finds little benefit to taking calcium plus vitamin D - basically no fewer fractures in the women taking the medication. I don't find this too surprising in that the dose of vitamin D was low - 400 units a day. It has been shown that it probably takes something like 2,000 units a day just to keep your level from going down. Furthermore, unless the subjects were getting a new bottle of the vitamin D supplement once a month or so, it is likely that it was decaying through oxidation, and not much was left in the bottles after a few weeks.
There was a higher incidence of kidney stones in the treatment arm of the study, so this is something to watch.