It appears the breast largely keeps fluoride out of breast milk.
Concerns have been raised as to very young infant exposure to fluoride in formula water and foods http://fluoridealert.org/studies/infant02/
This paper looks at the fluoride levels in milk, which were in comparison to levels in fluoridated water were low, (with the exception of the soy milk), which makes their conclusion thought provoking.
FLUORIDE CONTENT OF DAIRY MILK FROM SUPERMARKET
A POSSIBLE CONTRIBUTING FACTOR TO DENTAL FLUOROSIS
SUMMARY: Fluoride analyses were carried out on 42 different types and brands of milk obtained from supermarkets. The average fluoride content of dairy milk is 0.030 ppm, with a range of 0.007 to 0.068. Soy milk contains as much as 0.491 ppm fluoride. Infant daily fluoride intake as low as 0.04 mg/kg body weight can result in fluorosis of the permanent dentition. Therefore, in view of the very large variation in milk fluoride content, it is suggested that daily consumption of milk with high fluoride content could be a contributing factor to increased prevalence of dental fluorosis. In view of results of the present study, monitoring of fluoride content in dairy milk available from supermarkets may be necessary.