I've been reading the following site, created by The Vitamin D Council:
They claim that vitamin D deficiency is associated with a big stack of illnesses and disorders. There are links to several studies. They have a newsletter, and back issues are available on the site.
You may want to have your vitamin D levels checked next time you have your blood work done, especially if you live in an area that doesn't get much sun or you spend all day indoors pecking at a computer keyboard.