[cliche]I used to get a smacked backside, or a clip up the ear and it didn't do me any harm. [/cliche]
I think it is a natural way our species recognises the dominant members of the family group (IE our parents), as well as learning that doing X results in Y.
That said I do not think it is the only way children can be taught discipline but I think it can be effective - it certainly was for me.
My neices and nephews are growing up in the PC age where they get things explained to them, and they sit on the naughty step and suchlike. To be fair, they are all ill behaved compared to me and my brother and sister at the same age. I dont know if that is because of the 'soft' way in which they are disciplined, or because of other factors.
That said, I think smacking children opens the door for less moderate and less controlled parents to be violent and abusive under the guise of discipline.
Am I for or against? I am unsure. Generally against but I think some situations may arise where talking simply isnt enough. If I had a child who was about to use a brick on his newborn sisters head, I think he would get more than a talking to.