I've read through this whole thread and i am in total agreement with bilb. You really have to be a parent to get the whole picture here, not saying you cant have intelligent and insightful opinions but there is no teacher like practice.
I have 2 boys myself and have smacked my oldest (6yrs) when i feel its warranted (my youngest is only 4 months so he is safe for now). I know I'm not a bad parent and i know my kids are not unruly, smacking your child sometimes is the only way to get the point across IMO.
Obviously there is a huge difference between that and losing the plot and beating them black and blue, but common sense should tell you which one is which. If a parent cant tell the difference between beating and smacking then the ones who can shouldn't have to suffer.
Those of you who will not smack your kids when you have them i wish you luck.