I am opposed to corporal punishment in all forms. I believe it is completely unnecessary - with a bit of effort and imagination, parents can think of far more useful and constructive ways to shape their children's behaviour. Alternatives such as time-out, removal of privileges, and rewarding good behaviour are much more appropriate.
I also think that hitting a child serves to promote a culture of violence. It teaches children that their bodies are not their own; that adults have the right to inflict physical harm; that violence solves conflict; and that it is okay to bully someone younger and smaller than yourself.