Seems to me that smacking in anger is out because you may not have the control to temper the hardness of your smack, and this could actually cause serious injury. And smacking in 'cold blood' when you are calmer several minutes or more later (in a sort of 'wait till I get you home and then I'll smack you) seems to me to be gruesome! By then, you can reason and talk, surely. I think it should be made illegal, for children of all ages, as this sets the tone. It says that as a society we do not think smacking is an appropriate way to discipline children. In Denmark(or was it Sweden?), I think I read there have been no prosecutions for smacking, though one imagines they have laws against physical abuse, just as we have.
The case that provoked this was not the stick incident, but a teacher who pulled down the clothing of his nine-year-old daughter in the dentists waiting room to make her go for treatment. In fact, as she was nine, this would not be illegal under the proposed law.
I know there are times when a toddler will place himself in danger and you need to act quick! You remove him from the danger - why smack? Why is hurting a small child ok to make any sort of point? You can do just as much with a yelp or a grab away, and then explain, as far as you can, afterwards.
I smacked our eldest, once or twice, but I haven't done it with the other two, as I took a conscious decision that this was not something I wanted to do any more. I don't think parents who smack are awful parents - there are lots of horrible things you can do to kids that don't involve smacking, so not smacking doesn't make you any better than anyone else. But there is no evidence that it makes 'better' children, or that children who aren't smacked are 'worse'.