Spanking is physical abuse: it's not ok to do it to adults (illegality of physical assault/abuse) and even less ok to do it to children (when do by parents it adds an element of psychological abuse). End of story.
Spanking is included as physical abuse of children, in Sweden, which is illegal. I tihnk some other countries in the world have this law.
EDIT:
There seems to be a worries of a "bias" issue here on the thread, where people who were never spanked say 'no', and people who were said 'yes'.
When i hear/read "Spank", i think of a smacking that is aimed at the butt. Considering that, i was never spanked, but i was definitely "smacked" when i was younger.
When i was really young, my parents never laid a finger on me. As i got older (7+), i began to so what my dad considered "stupid" things (that were sometimes dangerous), and he decided to get my attention by smacking me across the back of the head. I'm not sure if he was going through something during those years (i have yet to seriously talk to him about this), but there were a few years when he did this with a certain frequency (7-10 years old). My mother clearly voiced her dislike of this, but she never physically attempted to stop him (only comforted me afterwards), maybe they were both going through some tough time outside of the family.
The frequency of this became very low as i entered my pre-teens (11-12), and eventually stopped before i hit puberty. i'm not sure if i did less "stupid" things (i still act without thinking, though with somewhat less frequency), or if my parents were no longer going through tough times, probably a bit of both.
My Dad hadn't hit me in over a year, when he decided to do it again (bad mood? I don't remember). Unfortunately, i was 13 and i knew from what my mother had been telling both me and my dad, that what he just did was wrong, and i wasn't going to stand for it. I smacked my fist straight into his chest, to get him to realize he needed to back off. Then i ran for it. He chased me a few steps and then smacked me hared than he had ever had before. I shouted. My mother, sister, and brother were appalled (my brother looked angry, while my mother and sister looked terrified) and started shouting too. I don't remember but i think he just left. My dad never laid hands on me again.
I love my dad, and we do NOT have a bad / abusive relationship. But this is something that actually happened, and something i still want to talk to him seriously about. He didn't exactly have an easy childhood, but even he knows what he did was wrong, just like he knows his smoking habits are bad for him (he's trying to stop smoking again, hopefully it will be for good this time).
I hope this helps to show that smacking children is fundamentally wrong, regardless if you experienced it or not. Someone on this thread made an excellent parallel: If hitting your spouse is wrong (and illegal), then hitting your children is wrong under the same logic.