>  There is a belief that breast-
>feeding prevents ovulation, thus preventing pregnancy.

Not completely, as I have seen learned discussions on the LeLeche groups
about how to eat properly when nursing one baby and pregnant with the
next.  Most LeLeche members are known for their intensive nursing, so
that might always be a factor, and might be more of a matter of personal
hormone profile.  

It does reduce the frequency of ovulation and although odds of ovulating
in any give month are reduced, they are not completely eliminated for
all women under all conditions.

Which basically means, "Well, yeah it does, but I wouldn't count on it
for birth control."  

And yes - any woman who has nursed before should be able to stimulate
milk flow again for an adopted baby, but it does take work.  It is even
possible for a woman who has never nursed or been pregnant as long as
she has completely matured, but this might require some chemical
intervention.  There is a drug for this purpose, a synthetic of
prolactin, but the name escapes me now. 

Fawn