There's an article that was posted recently on Science Daily about how much vitamin D a breastfeeding mother needs in order for some of it to be passed to her baby through her milk.

http://www.sciencedaily.com/releases...0606184845.htm

Summary:

The not-often-discussed issue of Vitamin D deficiency in nursing mothers is discussed by an expert, and how it can affect the infants in their care. An "adequate" intake for nursing mothers is not the 400 IU/d the IOM recommends, but is instead in the range of 5,000-6,000 IU/d, taken daily. If they get that much, they will meet not only their own needs, but their infant's as well.