No, it's not wrong, sometimes it just needs to be done.
Note the very, very important word in that sentence: sometimes.
It amuses me that in this thread smacking has been labelled as child abuse and even torture. I don't think any of us are talking about going all Gitmo on the kid (comparing a smack to water boarding is an assinine arguing point - it's hyperbole and bullshit, to be blunt).
I believe that to discipline an unruly child you warn them about their behaviour, if they continue then you opt for time-outs/the naughty corner/step, groundings, removing favourite toys/etc. You explain to the child what they did wrong, and (importantly) why it was wrong. Have you been raising them right (which you should be) then they will come and apologise, after which they can have their *whatever was confiscated* back if you deem them genuinely contrite (with small enough kids they are).
However there are times when children simply don't respond to verbal communication or dialogue.
This has been claimed to be "wrong" in this thread - but how many times have you seen a kid reach for the stove/fire, the parent says "don't touch that, it's hot" (as you can't smack their hand away, that'd be torturous) and then they touch said hot object and burn themselves. The infliction of pain in those scenarios is what teaches the child to not repeat the action (though if you want to argue that smacking a child is child abuse, then I claim allowing them to touch a hot object, which you can't restrain them from doing without it going under the same category, is neglect as you are allowing harm to come to a child in your care

).
Smacking is not about the infliction of pain. However to the child's brain it follows a similar path - a certain action led to an unpleasant consequence. It should, in my opinion, always be used as a last resort for when the child has repeatedly ignored warnings/groundings etc. or done something fairly hieneous. The smack is designed to capture the child's attention rather than inflict pain, allowing you to explain what they were doing wrong.
When smacking is used as a last resort for only the gravest of a child's misdeeds (I dunno, like trying to staple the family dog to the floor) and/or is a situation where they are likely to come to greater harm without the use of a smack; and this is combined with affirming good behaviour, incredibly important to raise well-rounded kids IMO, then it does not cause undue trauma nor induce fear towards a parental figure who should, in the child's eyes, be loving and "safe".
The confusion generated by a smack from a parent is exactly what makes the child not carry the same action out again. It is not fear of the parent, or fear of the consequence, but it is the memory of the displeasure of the parent (rather than the smack/"violence" itself) that causes the child to stop and actually think about what they are doing. This is why rewarding good behaviour (even if it is through simple hugs/stars and not treats) is vitally important in making smacking an effective, non-abusive punishment tool, as after all a loving environment is vital towards a child's development. However the once-in-a-blue-moon smack isn't going to harm the child - the short-term effect is often what is desired, particularly if the child is in a situation that will be highly harmful to them.
Now when I say "smack" I do mean a light tap on the wrist/bottom/back of the legs. It is not a beating in any sense of the word. Also, as you may have guessed, I endorse smacking as
ONLY a last resort measure, for either keeping the child from greater harm or for a punishment where a grounding/time-out will not suffice.
Were a parent to smack their child for every single act of disobedience then yes, I'd agree that it was wrong and abusive, and is almost certainly instilling fear rather than learning/respect into the child. But used as a final method in a household that opts for all of the "non-violent" methods of punishment first-and-formost I believe it to be absolutely fine.
After all, the proponents of the idea that smacking = child abuse comment (as evidenced by this thread) cite that non-corporal punishments are more effective and smacking isn't needed. Therefore they shouldn't have an issue of smacking being a last-resort option within a loving and caring family as the other methods will mean it's not actually needed, right? Banning smacking simply removes a tool in a parent's arsenal that can be incredibly important - namely from preventing the child from harming themselves, or another child, further. Thus I believe the option of being able to smack your child is more important than trying to label it as "child abuse" (after all, if a tap on the wrist/bottom is abusive, how do you rate an arm-wrench to keep them from dunking their brother's head in the pan of bioling water or running out into traffic? Has a much higher chance of harming the child, thus should be more abusive)- that designation should be reserved for people who actually beat thier children as a regular occurence and are, indeed, abusve.
Sorry for the wall of text - this topic annoys me greatly, as both sides display extreme...ignorance, almost (forgive me if it's not the right word). From people saying parents should "beat the little shits into next month" to others claiming "touching a child is abuse and torture!". Both have a slight point (however take it to extremes at either end of the spectrum), however both also miss the point-at-large, as I hope I have been able to elucidate in this post.