I have to agree that smacking, when used properly, probably does no more damage than other forms of punishment. The reason I can say this is that from my own personal experience as a child, I have several vivid memories of being punished by my mother and only one of them involves smacking.
I have one particular memory of being smacked by her, probably when I was about 5, and I had no idea why. I must've done something that she didn't like, but I had no clue what it was - so my shock/upset at being smacked for something that I didn't understand, has stayed with me. But I also have another memory of when she made me sit in the car while my brother and several other children played outside, and she would not let me get out. She was definitely angry with me, but again I have no idea what I did to incur the punishment - I just remember feeling very hurt by her treatment of me at the time. I also remember her being quite verbally critical/nasty to me at times when I didn't think it was warranted.
So what I am saying is, the times that I DID understand why I was being smacked, had no adverse effect on me, because I understood fully what I'd done wrong. I think it's only when a child doesn't understand what they've done and simply becomes the brunt of a parent's anger - in ANY form - that it does the most damage. I think it is just as damaging to isolate them/verbally attack them/lock them in their rooms/withdraw love & affection from them, as it is to smack them.
The child's understanding of the situation is of the utmost importance in my view, as is being shown love & acceptance by the parent following the punishment. I always try to explain to my son why I've disciplined him and make sure he knows I still love him anyway, no matter what he does.