The chapter by James McKenna and Nicole Bernshaw on "BF, Co-sleeping and
SIDS" from "Breastfeeding: Biocultural Perspectives" tries to unravel the
independent effects of bf vs. co-sleeping on SIDS (as bf and co-sleeping
often go together).  Also, the commentary in the book by Doren Fredrickson
briefly discusses his research on bf and SIDS, based on a huge sample of
U.S. data.  Fredrickson and his colleagues found that for each month of
exclusive bf, the SIDS rate is cut in half (so two months of exclusive bf
gives you 1/4 the SIDS rate of bottle-fed children, etc.; by the time you
get out to six months of exclusive bf there are basically no SIDS cases).
So far, Fredrickson's results are published only in an abstract from a
meetings paper, and in "Breastfeeding: Biocultural Perspectives."  Possible
mechanisms for the protective effect include breastfeeding's effects on many
of the different possible causes of SIDS, including:

bf reduces the danger of botulism poisoning, which is thought to account for
some SIDS deaths,

breast milk results in more rapid maturation of the brain (or ABM slows down
brain maturation, to look at it the other way) thus reducing the likeliehood
that immature brain development will lead to breathing problems

bf reduces the risk of upper respiratory infections that have been
implicated in some cases of SIDS

breast fed babies usually wake more often to feed, thus spend less time in
deep sleep cycles, in which apnea and failure to rouse are more likely to occur

and so on.

There are many postulated causes of SIDS, and breastfeeding is thought to
affect many of them, in various ways.

Kathy Dettwyler, Pres.
James McKenna Fan Club