"Are we (modern technological culture) effecting an eventual biological change, by rejecting 'nature' and turning to substitution for mother's milk? In other words, will we 'lose' the ability to lactate, as more generations of babies are not fed from the breast? In effect, 'use it or lose it'?"
That is a very good question talesin. It's the kind of question that reveals you have an understanding of how evolution works. What you're asking seems like a biologically plausible possibility but might require some other conditions for it to become a reality. Maybe if the human population got decimated and became very small and those who did not breastfeed had a survival and reproductive advantage over those that did, then yes it could happen.
Somewhat related to your question, it might interest you to know that there is a high incidence of lactose intolerance among East Asians. The reason for this is that they stopped drinking milk regularly like western cultures do and as a result, when a gene required to produce lactase - the enzyme used to digest lactose - became damaged, because there was no selective pressure to keep the healthy genome, the damaged genome became pervasive. There may have even been selective pressures to keep the lactase-deficient genome. For example, herding milk-producing cattle, might have slowed down a nomadic tribe fleeing from murderous pillagers or owning milk-producing cattle may have made the tribe targets of pillaging.
However, people in the west continued to rely on milk so there was a selective pressure to keep preserve the healthy gene and weed out defective versions that popped up.