A., you've received a lot of responses already, and some are conflicting, so I wanted to add a good website for factual information regarding this topic.
From kellymom.com:
http://www.kellymom.com/nutrition/vitamins/vitamins.html
<<Vitamin and mineral supplements are not necessary for the average healthy, full-term breastfed baby during the first year. Breastmilk is all that your baby needs for at least the first six months of life. Studies have shown that vitamins, fluoride, iron, water, juice, formula and solid foods are rarely beneficial to healthy breastfed babies during the first six months, and some can even be harmful. There are certain cases where a vitamin supplement may be needed for a breastfed baby during the first year, but these cases are the exception, not the rule.>>
As for Vitamin D supplementation specifically, check out kellymom's page: http://www.kellymom.com/nutrition/vitamins/vitamin-d.html
It goes into great detail about whether to supplement the baby or the mother, or just give sun exposure, etc, depending on your baby's individual level of risk.
And FYI, I never supplemented my bfing baby, and she was exclusively breastfed until about 9 months (and very healthy).
HTH, M.