Google says:

breastfeeding was found to raise intelligence an average of nearly 7 IQ points if the children had a particular version of a gene called FADS2.

Later:

Ninety percent of the children in the two study groups had at least one copy of the "C" version of FADS2, which yielded higher IQ if they were breast-fed. The other 10 percent, with only the "G" versions of the gene, showed no IQ advantage or disadvantage from breastfeeding.
The gene was singled out for the researchers' attention because it produces an enzyme that helps convert dietary fatty acids into the polyunsaturated fatty acids DHA (docosahexaenoic acid) and AA (arachidonic acid) that have been shown to accumulate in the human brain during the first months after birth.
Since the first findings about breastfeeding and IQ appeared a decade ago, many formula makers have added DHA and AA fatty acids to their products.

Basically, it's one of those cases where doing it is probably better than not, but not at all costs. If you are constantly stressed because your baby prefers the bottle, or you get repeated breast infections, or you don't produce enough milk, it's time to direct your efforts elsewhere.