I don't agree. I recognize that this is a difficult and controversial issue, but perhaps you would allow me to present a contrary opinion.
(Also, understand that this is a general response, not a response targeted only at gnaritas).

A few years ago, the New Zealand legislature passed a law that effectively made spanking a punishable offense(^), which generated a lot of discussion about the topic. About two-thirds of everyone I talked to (classmates, etc) had be spanked as a child, and of those people, I never met anyone who considered it a bad thing or traumatic in any way. In fact, everyone recognized that they had only been spanked because they were misbehaving, and that they had 'earned' the punishment. Let me emphasise that: I have not talked to anyone who has been spanked, and who thought it was traumatic or equivalent to violence in any way.

From this, I suspect that most people who object to spanking have either never been spanked (and assume it would be tramatic), or have parents who thought 'discipline' was an acceptable excuse for abuse. Please understand that I am not defending any parent who abuses their child. Their actions are horrid and without excuse. Instead, my point is merely that spanking, done right, is not at all equivalent with child abuse.

So what makes a 'good' spank? It should be short, sharp, and clearly associated with the crime, not the person.

By short and sharp, I mean that it should hurt but I should not leave any long term effects. There should only be a single strike, the pain should not last longer than 10 seconds, and if there is a bruise then the parent has seriously stuffed up. The purpose is not to inflict pain on the child, but rather to clearly and unambiguously tell the child that certain actions are not acceptable. A baby should never be smacked, because they would not be able to understand the causality, and I would expect that once a child is over the age of 5 or 6, a verbal rebuke or simple smack on the hand should be sufficient. (Note that the smack on the hand would not be designed to cause pain, but rather to serve as a strong rebuke.)

The smack should be clearly associated with the crime, not the person. When my mother smacked me, she would call me into the bedroom, explain what the crime was, smack me once, then hold me as I cried. Also, she always told me that she still loved me.

Gnaritas: you say that the end does not justify the means. I'm not sure I agree with that. If, by being disciplined while I am a young child, I can learn to behave as a civilized member of the human race, I think this is a very good end that does justify the means. I suspect that the only people who can discipline themselves without any external discomfort (whether that's physical pain, boredom from being given a detention, or embarrassment from a boss's rebuke) are those who have been appropriately disciplined when they were young. Certainly, parents should move to the milder forms of punishment as soon as a child will respond to them. But while a child is young (3-5ish), a short sharp smack is an effective tool that doesn't need to be abusive.

I recognize that some people will read this and still thing smacking is horrid. That's okay; you're certainly allowed to form your own opinion. However, please do not make the mistake of assuming that smacking must be abusive or violent, or even that the children themselves resent it. I know I never did. I disliked the pain, but I knew that there was a very good was to avoid a smack -- stop being mean to others around me.

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^ The purpose of the bill was only to remove a legal defense from those who were 'obviously' Bad Parents, but the ambiguity about what constitutes a Bad Parent made it very controversial.