1) Many people (me included) have a strong, emotional aversion to the thought of using physical force against a child that has effectively no way to defend themselves. (Needless to say, such emotional reactions say nothing about whether it's actually right or wrong to hit a child .)
Indeed, I don't have any such aversion, and tend to score pretty low on the "emphathize with other human beings" thing. Which probably explains some of my puzzlement :)
2) AFAIK, there are studies indicating that spanking is an ineffective method of discipline. In general, it seems that positive reinforcement has stronger effects than negative. That'd lead to spanking being both useless and needlessly hurtful to the child, and therefore obviously something to be avoided.
I definitively will research the subject more before I have kids :) Overall, I'm dubious about the idea that negative reinforcement (a.k.a. punishment) is fundamentally ineffective, since fines and jail sentences do seem to work as a deterrant.
3) The notion that hitting a child to make them do what you say teaches them that it's okay to use force against others to get what you want.
Depends how it's used. As the guy in psychohistorian's link says, the problem is when the severety of the punishment depends not of what the kid did, but of how the parent feels. If the kid gets spanked when he knows he did something wrong (like lying), he shouldn't interpret it as meaning that "it's okay to use force against others to get what you want."
See also my recent review of Kazdins positive reinforcement method:
http://lesswrong.com/lw/jzg/book_review_kazdins_the_everyday_parenting_toolkit/
People debate all the time about how strictly children should be disciplined. Obviously, this is a worthwhile debate to have, as there must be some optimal amount of discipline that is greater than zero. The debate's nominal focus is usually on what's best for the child, with even the advocates for greater strictness arguing that it's "for their own good." It might also touch on what's good for other family members or for society at large. What I think is missing from the usual debate is that it assumes nothing but honorable motives on the part of the arguers. That is, it assumes that the arguments in favor of greater strictness are completely untainted by any element of authoritarianism or cruelty. But people are sometimes authoritarian and cruel! Just for fun! And the only people who you can be consistently cruel to without them slugging you, shunning you, suing you, or calling the police on you are your children. This is a reason for more than the usual amount of skepticism of arguments that say that strict parenting is necessary. If there were no such thing as cruelty in the world, people would still argue about the optimal level of strictness, and sometimes the more strict position would be the correct one, and parents would chose the optimal level of strictness on the basis of these arguments. But what we actually have is a world with lots and lots of cruelty lurking just under the surface, which cannot help but show up in the form of pro-strictness arguments in parenting debates. This should cause us to place less weight on pro-strictness arguments than we otherwise would.* Note that this is basically the same idea as Bertrand Russell's argument against the idea of sin: its true function is to allow people to exercise their natural cruelty while at the same time maintaining their opinion of themselves as moral.
One example of authoritarianism masquerading as sound discipline (even among otherwise good parents) is the idea of "My House, My Rules." I've even heard parents go so far as to say things like: "it's not your room, it's the room in my house that I allow you to live in." This attitude makes little sense on its own terms, as it suggests that parents would have no legitimate authority over, say, a famous child actor whose earnings paid for the house. Worse, it's a relatively minor manifestation of the broader notion that the child has a fundamentally lower status in the family just for being a child, that they deserve less weight in the family's utility function. I don't think this is what parents would be saying if recreational authoritarianism really were not a factor. They would still say that they, by virtue of their superior experience and judgment, get to make the rules (i.e., decide how to go about maximizing the family's utility function, though even this might be done with more authoritarianism than is necessary). But you wouldn't be hearing this "I'm higher than you in the pecking order and don't you dare forget it" attitude that is so very common.
*Some might argue that arguments should be evaluated solely on their merit, and not on the motives with which they were offered. This is correct when the validity of the arguments can be finally determined. For most kinds of persuasive argumentation, especially in complicated and emotionally laden subjects like child rearing, arguments work on us without us ever being able to fully evaluate their merit. And in that world, it does make sense to down-weight arguments that have some bias built into them.