Thanks for correcting my reference to Singer as the founder of "animal liberation." Though it was his 1975 book of that title that triggered the popular movement, he did get his ideas from conversations at Oxford with the Godlovitches, to whom he expresses gratitude in the preface. And shouldn't we also give some credit to Jeremy Bentham, the founder of utilitarianism and first radical thinker of the moral orthodoxy, as the ur-animal-liberationist? In 1789 Bentham wrote, "The French have already discovered that the blackness of the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor. It may one day come to be recognized that the number of the legs, the villosity of the skin, or the termination of the os sacrum are reasons equally insufficient for abandoning a sensitive being to the same fate. ... The question is not, Can they reason? nor, Can they talk? But, Can they suffer ?"
Back to infanticide for a moment. You say that there is nothing in Singer's utilitarian position that would rule out the (painless) killing of newborns for "trivial kinds of reasons." First, let's consider this from a practical view: Who would ever want to do such a thing? Even King Herod had a non-trivial motive for his slaughter of the innocents. And lest someone try to out-Herod Herod, it should be pointed out that in Singer's view only a newborn's parents would have the right to kill it for any reason. Now, with legal abortion available, it would take a real masochist of a mother to wait until she had already been through childbirth to have her baby destroyed for reasons of convenience. This seems to happen mainly with troubled teenagers, whose judgment takes cognizance of neither moral principles nor legal sanctions. And it is wrong on utilitarian grounds, as long as there are barren couples who would be gladdened by the opportunity adopt the unwanted baby.
In some societies, however, there are normal newborns who are absolutely unwanted. Infanticide of healthy female babies has certainly gone on in China and India, and the consequences of this practice were pointed out years ago by Amartya Sen, the new Nobel laureate in economics, in an important article in the New York Review of Books titled (I think) "One Hundred Million Women Are Missing." But even in these societies infanticide has almost been eliminated as prenatal genetic testing has become available. The deficit of baby girls that has developed in northern India since the 1980s is due not to infanticide but to "eugenic" abortion of fetuses with two X chromosomes (which is also an option in this country). Is this moral progress?
Despite the empirical rarity of convenience-infanticide, which is opposed by every impulse of evolutionary psychology, Singer is clearly troubled by it, leaning toward the view that "for legal purposes, since birth provides the only sharp, clear, and easily understood line, the law of homicide should continue to apply immediately after birth." He countenances euthanasia as an option only for incurably ill and severely disabled newborns. And it remains true that a baby just out of the womb, like a 6-month-old fetus, is sentient without being a person. It has no sense of itself as a being with a future; no future-oriented desires that could be thwarted; hence, it cannot be said to have an interest in its continued existence. It acquires such an interest only retrospectively, if it survives. But the adult who develops from the neonate cannot remember being the neonate, and has no mental links with it--nor with the fetus.
Singer conscientiously works out his conclusions in a framework of preference-utilitarianism. For him, the ethical data are actual desires and sufferings. Something is prima facie wrong if it frustrates desires and causes suffering. For you, Colin, it appears that something is prima facie wrong if it offends a strongly held moral sentiment. Infanticide is "repugnant," it is wrong. You deduce this reason from a moral absolute--the sanctity of the life of a person--and what seems to be an ad hoc metaphysical theory of "degrees of potentiality." (Opponents of legal abortion have a different theory of potentiality and different moral sentiments.) But where is your moral framework?
Utilitarianism is vulnerable to objections, but in Singer's version it is about the best moral theory we have. The standard philosophical criticism of it is that it is too rigorist--it demands that we practically sacrifice our own interests for the happiness of humanity. Singer, too, is a moral rigorist, not a slackard. He thinks we shortchange the interests of suffering people in the Third World in our moral calculus, and that we cause undue suffering to animals--and that we torture and kill higher animals who are, philosophically speaking, persons, though not human persons. What do you think of Singer's "Great Ape Project"?