I think I was spanked probably 4 times in my life. And I remember every one. Each time it was because I was warned not to walk into the street, which I for some reason liked to do. I must have been 4 or 5, and it's still clear as day to me. Warning, warning, then I just charged right out after the city bus.
With my own baby (a month old now), I hope I'll never have to. My wife is a teacher and really good with discipline/high expectations so I'll probably follow her lead. I might swat his rear 3 or 4 times in life should he run into traffic, but I really hope I won't have to... as a parrent now, I can see how that woudl huurt the parent, more than the child.