Yes, I also strongly disagree with the smacking. What I want most for my child is for him to grow up happy, secure, respectful, and at peace. If I don't treat him with respect, how is he going to learn to be respectful? If his parents, the people with whom he has the strongest relationship and who are most responsible for him, hit him when he does something wrong, how can he grow up secure?
This fits right in with disciplining a toddler. My toddler is developmentally at a stage where he is testing the boundaries. He wants to know what my reaction will be if he does X, and whether he can sometimes get away with it, or never. If he hits me or someone else, and to teach him that it's wrong I hit him back, what message does that send? It sends the message that you are allowed to hit, but only if you have the power in the relationship. It also tells him that I don't respect him very much. I would never think of hitting my mother, or the guy sitting next to me on the train, or some other kid... why is it okay to hit my own kid?
He is not doing this out of malice. He is a good kid, who is fairly new in the world, and who is trying to figure out what's acceptable and what's not. So hitting him doesn't project the message I want to project: that hitting is not socially acceptable behavior. The way to do this is to calmly, respectfully, tell the child when some behavior is not acceptable. If the behavior continues, it's not because the child is a little snot. It's because he is testing to see what my reaction will be next time - he's still trying to learn about the world. If my reaction next time (and the next, and the next) confirms what I've already told him, that his behavior is unacceptable, he will eventually realize that the behavior is always unacceptable. He'll never realize it if the messages I send contradict each other (i.e. "hitting is wrong" *smack*).