Vitamin D has come up a few times here before, and it's understandably a big concern for raw foodists, who do not eat food that has been supplemented with D.
I certainly respect the opinions and expertise of the other contributors, but I feel that D supplementation is being oversold without regard to what the consequences may be.
There's significant reason to believe that D supplements are not effective in preventing problems associated with D deficiency, and that they bring on additional medical problems themselves, as is often the case with vitamin supplements of many types as well as drugs and cures in general.
I think most people are unaware of just how much evidence against D supplements is out there, and that experts are starting to shy away from the supplement-happy suggestions they were issuing before.
For an in-depth look at the Vitamin D supplement issue, check out my article here: http://www.raw-food-health.net/Vitamin-D-Toxicity.html 
Happy sunning :)