Breastfeeding also lowers the mother's susceptibility to breast cancer, and (statistically) increases the IQ of the child.
For maximum effectiveness, the baby should be breastfed for over a year.
some links:
http://www.breastfeedingnetwork.org.uk/index.html
http://bmj.com/
search for breastfeeding + IQ
i had the link here before, but it was several hundered chars... :D