I'm actually really torn on this one.
I absolutely agree that slapping or spanking can really send the wrong message, but I think there might be some occasions where it might be warranted.
I remember my younger brother went through this phase when he was 2.5 years where he would go for a walk out of the yard, down the street, and we'd find him blocks and blocks away.
My mum watched us like a hawk, so the fact that he managed to slip away still amazes me! Anyway, he did this once and our entire family was frantic looking for him - mum reported him missing to the police and everything. Luckily, we found him and he was fine. Mum sat him down and had a big talk with him about why what he did was wrong,etc.
Then she caught him trying to go for another walk again a few days later and again, stern reprimands, sent to his room, the whole bit.
A few days after that, he slipped away after dinner and once again, we were all frantically looking for him. He was found within minutes, even though he had already made it several blocks away (at that age, kids can move!). Mum gave him a sound spanking and he finally got the message.
He never did that again.
Was it justified? Maybe she should have stuck with the naughty step and time-outs, but this was behaviour that was putting my brother in danger and he wasn't listening until the spanking sent the message home.
I don't judge her for that. In fact, I'd be pretty tempted to do the same thing.
Several of my friends say that that's what they have saved any physical correction for: a serious message about dangerous behaviour (running into traffic, etc).
I think that might be the big difference, too. We got spanked for really serious things, not smacked around all the time. So a spanking was a big deal and it usually got results.
I don't advocate this for much besides the kinds of actions I've just described, but I think I'd want to be sure that my kid was safe and that can sometimes be a stern lesson.
But this is all theory for me. I wonder if I'll be able to use that kind of discipline when baby is out and finally a real little person to deal with?