I don't see much of a moral difference between a kid hitting his parents when they don't do what he wants and parents hitting their kid when he doesn't do what they want - but I do see a difference between those and spanking your kid when he did something wrong and he knows it (for example : hitting a younger child for fun, stealing candy from the store, burning his schoolbook then lying about it ...).

By the way, I did know a kid who would threaten to hit his mother. What a brat he was, he's probably why I'd rather have a bit too much disciplin than not enough.