As a former employee of a pump company, I have to put my .02 in here:
Because the mechanics of a baby removing milk from the breast include jaw
compression, and the mechanics of pumping rely solely on nipple stim to
initiate letdown and then suction removal of milk, is it similar or not?  My
guess is that the baby will always be more efficient at milk removal, but
pumping to stimulate and remove milk will maintain milk production
adequately.  When I worked in a hospital as LC, we always noticed a decrease
in supply of moms pumping for premies when baby had not been to breast yet.
 Also, the flexible shield offered by one or two pump companies is marketed
as "increasing prolactin levels thus allowing greater milk production."  Now
don't quote me verbatim because there are several generations of new pumps
out that I have no experience with.  But the original concept of pumping is
as described above - and I will visit all the companies at the conferences
this summer to learn more.
Thanks for listening
Mary Kay Smith, CLE, IBCLC
Romeoville, IL