In Canada, the Canadian Pediatric Academy recommends 10mcg/day or 400UI/day.
There as been quite a few studies about the subject and a recent 1994 one (I
can't remember the name of the author but I have it at my office) recommends
that breastfeeding babies do not need supplementation.

As I am Breastfeeding Coordinnator for the Public Health Center in Quebec,
and because I have to work with these recommandations, I try to emphasize
the fact that the recommandation is a preventive measure. Some babies are at
risk of Vitamine D deficiency: babies that live in a nordic country
                       babies with dark skin
                       babies that are not exposed to sunlight
                       Use of sunscreen lotion also in an element to
consider as                        we have a proble with our ozone layer.

As a certified lactation consultant I do beleive breastmilk is the only
nutrtion a baby needs and should not, in normal circomstances, be
supplemented.