The results of this quest for the true story of this wonder vitamin have left me stunned. Modern medical thinking (mainstream medicine) now says that vitamin D has a spectrum of positive effects on the human body from bone health (we already know that), cell membrane integrity, autoimmune diseases, flu, heart disease, liver disorders, and more. In fact, diseases like diabetes are also strongly linked to vitamin D deficiency. One leading authority (in Cochin) on the subject went so far as to say that deficiency of this vitamin (cholecalciferol-D3) can be linked to almost 80 % of known human diseases, pathology. These 2 you tube videos spells out just that. Many modern medical experts now believe that even most forms of cancer from breast, prostate and the colon can be prevented (to a large extent) by a daily dose of this vitamin. See this short video by John Cannell, MD.
(Also see this link for some basics of Vitamin D metabolism: http://en.wikipedia.org/wiki/Vitamin_D )
Other links that might be useful: http://www.ncbi.nlm.nih.gov/