When the spanking is so hard that it causes physical damage, or intense, searing pain. It should be a short, sharp, shock to the system, that's all.
Also, when physical reprimands are without pre-emptive warnings and an explanation of why a certain behaviour was 'wrong' in the first instance.
I think a spank on the back of a hand or the bottom is reasonable if the child wilfully refuses to respect reasonable rules and or is purposely and repeatedly doing something that will endanger his/herself or another.
I think that physical reprimands should always be a last resort though. If Supernanny has taught us anything, it is that a naughty child is the result of an impatient and ignorant parent, who has not consistently enforced and rewarded good behaviour.
There are highly effective non physical techniques, such as time-out, removal of favourite items, grounding, reasoning, rewards, and other even more creative methods, that, unless in an emergency or urgent context, should be used before striking a child.