You describe Peter Singer as the "founder of the contemporary animal-liberation movement." Let me put the record straight on this. In 1972 John Harris and Stanley and Roslind Godlovitch published Animals, Men and Morals, which articulated the principles of the contemporary animal liberation movement (of course this book itself had many antecedents). They were philosophy postgraduate students in Oxford at the same time Singer was. Singer was greatly influenced by this book (as was I) and famously published an unsolicited review of it in the New York Review of Books soon after its publication. He then published Animal Liberation in 1975, which expands the themes of his review. The substantial intellectual content of that book is almost entirely derived from the earlier book he reviewed. Animal Liberation became a huge success and effectively eclipsed the earlier work by the contributors to Animals, Men and Morals.
I mention all this not to detract from Singer's own book, for which I have nothing but the highest praise. But I do think it is worth noting that these ideas did not spring whole from Singer's mind (not that he has ever claimed they did) but had their antecedents in the work of other people, who should be given far more credit than is customary. John Harris abandoned philosophy to do social work, Stan Godlovitch is a philosopher at the University of Colorado at Boulder; and I don't know what became of Ros Godlovitch. In any case your description of Singer as the "founder" of this movement is highly misleading. He was an early follower and brilliant expositor of a philosophically worked out position developed by others.
I'm afraid, too, that you have misunderstood Singer's position on infanticide, as well as my own negative reaction to it. The point at issue is not whether infanticide is permissible in cases where the future life of the baby will be intensely miserable or even totally comatose. There is room for sensible people to disagree here, and I am certainly not saying that I am repulsed by the suggestion that under some circumstances it might be better to allow such an unfortunate baby to die. The trouble is Singer's attitude toward the killing of normal healthy babies. Remember that he is in favor of abortion for relatively minor reasons (as that the baby would interfere with the mother's career plans) even in cases where the fetus is perfectly healthy. He then concludes from this, given his diagnosis of why the fetus has no right to life--that it is not a "person"--that the same thing must be said of the infant. Hence the infant could in principle be done away with in essentially the same circumstances as the fetus. (He mentions population control, apparently approvingly, as one of the reasons some cultures have practised infanticide.) Perhaps the most chilling moment in Singer's sketchy and distressingly unengaged treatment of infanticide is this choice passage: "None of this is meant to suggest that someone who goes around killing babies is morally on a par with a woman who has an abortion. We should certainly put very strict conditions on permissible infanticide; but these conditions might owe more to the effects of infanticide on others than to the intrinsic wrongness of killing an infant." (my italics)
Notice that we are discussing normal infants here, not severely impaired or suffering infants. What Singer is apparently saying is that the only problem with such infanticide lies with the effects on others--there is nothing wrong with it so far as the infant is concerned. If the infant had no one that would grieve at his or her death, Singer can see no reason to morally oppose infanticide--for reasons of population control, maybe, or simply because the child would become a burden on the state. This is because, for Singer, an infant has no intrinsic right to life, since it is not rational, self-conscious, and aware of its own past and future. This is what I said was morally monstrous; not mercy killing but convenience-killing.
You are thus quite wrong to suggest that Singer's generally utilitarian philosophy leads him to oppose the killing of healthy infants for trivial kinds of reasons. For the same kind of utilitarian argument would lead one to condemn the abortion of a normal early-term fetus--it too could lead a happy pleasurable life if nature were left to take its course. Singer blocks this unwelcome consequence of utilitarianism by requiring that the individual in question be a full-blown person: But then, as he recognizes, he has a problem explaining what is wrong with infanticide--a problem he boldly embraces as a morally sound conclusion.
The right thing for Singer to say at this point is this : "OK, I never liked that conclusion anyway, I just thought it was forced upon me by my liberal stance on abortion. I now see that I misdiagnosed the abortion issue, as Colin McGinn has so generously pointed out. I therefore gladly reject my earlier position on infanticide. Truth to tell, it always made me morally queasy."
Can I end on a personal note? I had always admired Peter Singer as a moral philosopher, and I expected that when I looked at his Practical Ethics I would find all the fuss about his appointment at Princeton to be based on misunderstandings and prejudices. But I have to say that I was genuinely shocked by his treatment of abortion and infanticide: both by its philosophical naivete and its repellent moral content. I encourage you, Jim, to study the relevant sections more closely; your reading of the text is far too charitable and colored by your own right-mindedness.