Here are a couple studies. There are lots more and, of course, they don't all agree. There was one study that suggests that the effects vary depending on genetic differences in fatty acid metabolism pathways. Anyway, I used Entrez Pubmed and typed in "IQ Breastfeeding".

http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1718901&blobtype=pdf

http://www.bmj.com/cgi/content/full/333/7575/945