Bloss, I found it interesting that you told your son that hitting mummy is wrong, and to teach him this you hit him! I'm not having a go, I just found it ironic!
I too was hit by my mum and I have inherited her short temper, which makes me capable of lashing out instead of taking time out. I have lashed out at my dd once, and I felt extremely bad about it. But this is the difference, I had done wrong, I had lashed out without thinking, a lot of people here have thought about it first. However I don't think that makes it particularly right.
Any child under the age of 2 is still a baby by my reasoning, they do not understand right from wrong. When they test you, they are just finding out their limits, they are learning what is acceptable and what is not. So I don't think at this age they are capable of being deliberately naughty and so I don't think they deserve to be smacked. However if you put me in the position that Batter's friend was in, when her child ran across a busy main road, I might well go for the sharp short shock treatment, although I think my reaction to them would probably suffice.
I guess I am lucky that dd has never done anything that naughty. Usually if I shout her full name at her very sternly she stops what she is doing. If she doesn't I will go and physically remove her, look at her and tell her off. I never have to do more than this.
I can understand why people here feel they need to smack, but I don't really agree with it, not for kids under 2. Seems to me that the people I know who do smack their kids, have very naughty kids, perhaps because once mummy has used her ultimate form of punishment, there is nothing worse to come.