Jonathan:  You are right about the fact that the most likely time of
transmission of HIV through mother's milk is the time when she has a lot
of virus particles floating around in her bloodstream.  There are two such
situations:  1). The mother becomes infected while breastfeeding.
             2). A previously HIV+ mother's immune status becomes so
        compromised that she has viremia (usually someone with more advanced
        disease)
That's probably the reason why there is higher transmission through
breastfeeding for longer than 6 months in populations with no access to
antiretrovirals.  We have to remember that the studies being quoted come
from countries where medical care and availability of medications is
limited.  I haven't seen any data on transmission in mothers on
antiretrovirals with low or undetectable viral loads.  The studies have
probably not been done.  Hope this clears up some of the confusion.
Regards, Alicia Dermer, MD, IBCLC.