First I have to say that my daughter is only 7 months old now, so I don't have first hand experience (pun intended) with punishing her yet, but naturally, it's a topic I've been considering for a while, put a lot of thought in and did quite some reading on.
In general, my opinion is that physical violence against a child is generally a bad thing and should be avoided if possible. That said, I don't think mild physical punishment is necessarily a big deal or even abuse, as long as the context is right. Most likely, mild spanking is easily understood by a child, as long as it comes together with communication and explanation, and the child realizes well it has done wrong. While on the other side, even very mild spanking that hardly ever hurts but just shocks the child, may be very traumatizing when the child doesn't understand the reason and is left alone with this shock.
But as a matter of principle, I'm rather comfortable with a zero tolerance policy towards spanking, because it's a slippery slope and the line to abuse is very thin. Basically all parents who abuse their kids start with "just spanking", and basically of them think "I'm just spanking a little". Also, I don't think spanking should ever become a habit for a parent, or even worse, serve as a means to vent the own frustration. Spanking out of that motivation, rather than a means that focuses on the learning effect in the child, is *always* bad and there is no excuse, IMO.
It's also a huge misunderstanding, IMO, that discipline requires physical violence, and absence of spanking equals lack of discipline. I've seen enough examples of very disciplined children who have never been spanked, and on the contrary, kids who have often been spanked and then act even wilder, at least when dealing with other people than their spanking parent, out of spite and to pass on their frustration. There are other and better means of creating discipline than spanking, and I'm pretty sure they work well with most children.
ALso, there is the danger that children learn the wrong lesson from spanking (especially spanking that is not accompaigned with sufficient communication): That it's okay to use physical violence against those who are weaker to get your will. The worst schoolyard bullies I met in my life were all children who were excessively spanked by their parents. They had to regain their sense of strength and self-confidence by scoring victories over weaker children.
When it comes to the different theories I read about, it seems that the according literature sees three main branches of education styles: The authoritarian education, the anti-authoritarian and the democratic education.
Physical punishment is usually a sign of authoritarian education; the parent is a tyrant, a dictator who has to be obeyed as a matter of principle, it must never be questioned and the child has no or few voice in the decisions the parent(s) make. Even worse than disobedience is questioning the parent's authority, and this alone is punished. This style of education breeds authoritarian and either passive or violent characters, and it is associated with many personality disorders in adult life.
The extreme opposite is anti-authoritarian education: The child is supposed to develop entirely naturally and is given no limits or rules. Problems naturally arise when such kids start violating the needs and limits of other people. Problems also arise regarding discipline and social behavior.
Often lauded is a democratic education style, which looks like a sane middle way: The child is given a well defined, broad leeway inside which it may realm independently, but the limits that do exist must be strictly enforced. Rules all agree on are a matter of negotiation between parent and child, while of course the parents maintain the last word, but not without first listening to the concerns and opinions of the child, and proposing compromises when possible. It is vital to teach the child empathy, and to respect the needs and limits of other people, in exchange for other family members respecting the needs and limits of the child.
Perhaps more important than the question whether mild spanking is okay, is the question about the appropriate style of education. When spanking takes places, either deliberately or "accidentally" out of an extreme situation, the context is probably more important than the spanking itself.
So far my idealistic ideas today. But hey, ask me again in 3 years or so.