The merits of Vitamin D supplementation are well known and it has become a common practice in many parts of the world.
What remains to be clarified is what is the optimal blood levels of Vitamin D and what are the optimal co-factors (there remains debate about if the Weston A. Price foundation is using too much Vitamin A, and they would suggest others are using not enough).
Too high levels of Vitamin D may be linked to the development of kidney stones (for example, see http://www.vitamincouncil.com/newsletter/2006-aug.shtml).
Kristine Ensrud et al. Circulating 25-hydroxyvitamin D Levels and Frailty Status in Older Women. Journal of Clinical Endocrinology & Metabolism, December 2010
found that both lower and higher levels of Vitamin D were associated with increased frailty.
Like most things in life, they should not be considered in isolation, nor should they be used in excess.
A reassuring study came out 3 days ago from the Mayo Clinic. Read about it at Science Daily: Vitamin D toxicity rare in people who take supplements, researchers report
Americans have low vitamin D levels, research shows, and as a result, vitamin D supplement use has climbed in recent years. Vitamin D has been shown to boost bone health and it may play a role in preventing diabetes, cancer, cardiovascular disease and other illnesses. In light of the increased use of vitamin D supplements, researchers set out to learn more about the health of those with high vitamin D levels. They found that toxic levels are actually rare.