Well, I've had mild smacking and spanking and there is definitely no harm in doing so. Then again, both instances were a rarity and only when I was being defiantly disobedient.
As others have said, there is a clear line between discipline and abuse. Trying to define specifically that line will only lead to parents "accidentally" crossing the line and much confusion and dispute. So the best thing is to forbid it altogether so there is no temptation to over-discipline your child.