I have read and watched an awful lot of stuff about child disclipline, being the parent of a 6-year-old child. As far as I have read and seen, the advice is that a hit should only be used in the follow circumstance: When a child is posing an immediate threat to the safety of themselves or others. For instance, if a child on repeat occasions runs towards the road and does not pay attention to a verbal "STOP", or if they are acting in an un-safe manner towards other kids and do not stop when told. And if a hit is used as discipline in this instance, it must be clearly explained to the child WHY they were hit.
As far as my own opinion goes, there are plenty of ways to disclipline a child without violence. If a hit was used for as a day-to-day disclipline in every instance of misbehaviour, the child would be constantly anxious, and would be learning that the way to solve problems with violence. Also, I know from friends that have been hit as children that quite often it wasn't about discipline at all, but rather their parent losing their temper and taking it out on them.
I have found time-out to be ridiculously effective. You give the child a warning: "Stop doing or you will go into timeout". This makes the child have to choose between carrying on the bad behaviour, which results in a punishment, or switching to good behaviour. In effect, you are teaching the child self-discipline. And lets face it, a three year old does not want to have to sit still for three minutes. They would probably rather be hit! This worked so effectively for my son from the age of two that pretty soon this translated into his doing what I told him without having to threaten timeout (as is necessary in public), because basically it had established the fact that Mummy and Daddy are the bosses, and you do what they say.