As Jothar noted above, the primary incentive for milk producers to substitute traif milk for kosher would be economic; that is, if it were cheaper for producers to sell pig milk as kosher, they might do so. However, in the United States most milk is produced on large dairy farms, which rarely have any pigs. Even if they did, automatic milking equipment is sized for cows, not pigs. Also, it seems to me that pigs don’t take to milking very well (although since I don’t know much about farming I might be wrong about this). In fact, since pigs are raised mostly for meat, pig milk is not generally commercially available in this country.
Goats and sheep are easier to milk than pigs, but these types of milk tend to be more expensive than cow’s milk, and can be sold for more money, either as liquid milk or in the form of specialty cheeses. Therefore, there is little financial incentive to substitute these for cow milk. Like pigs, goats and sheep wouldn’t fit well in the milking equipment. Most importantly, even if goat or sheep milk were passed off as cow milk, the resulting product would still be kosher.
Chinese manufacturers substituted melamine for nutrients in baby formula because there was a financial incentive to do so. As Jothar noted, even the prospect of harsh penalties did not deter them; indeed, two people have already been sentenced to death for this. If there were a financial incentive for milk producers to substitute milk from treif animals for cows’ milk they might do so, but given the realities of dairy milk production in this country these incentives don’t exist.