I think the IQ study was only done on preemies.  The logical assumption
would be if it helps fragile, at-risk babies it would work even better on
full-term babies.  But I'm pretty sure there haven't been any formal
studies.

Lactation and a reduced risk of premenopausal breast cancer by Newcomb et
al, New Eng J of Medicine, Jan 13, 1994, vol.330, No 2, p. 81-87.

Independent protective effect of lactation against breast cancer: A
case-control study in Japan by Keun-Young Yoo et al.  American Journal of
Epidemiology, Vol 135, p. 726-733.

There are quite a few. Check Lactnet archives.  Some say yes, some say
maybe, some say no dif.  The best list of articles would probably be
available through the LLLI Center for Breastfeeding Information.  They will
print out a list of pertinent articles for a nominal fee.

Sincerely, Pat Young in SNJ, USA