just to clarify, i don't think that if you spank that means you've failed as a parent, i think it means you've failed as a parent in regards to a specific situation. and that's totally understandable -- parents aren't perfect, they're human, and the important thing is that they do the best they can, acknowledge when they screw up, and remind the child that they are loved "always in all ways" -- actually, my parents wrote that phrase on my high school graduation card and i nearly cried ... seriously, parents, it can be difficult to talk to your teenagers, so write them little letters and cards on special occasions -- and that there was a reason for the spanking, and that it was a direct result of an action by the child.
of course, such a conversation would probably have to happen after the fact, when everyone has cooled down.
at the end of the day, i think spanking isn't a good thing, but it's an understandable thing, and children are smarter and more resiliant and more perceptive than we usually give them credit for. and this goes both ways. if a parent is spanking out of frustration, the child will sense that. if the parent is spanking out of some perverse pleasure of being able to dominate a child, the child will sense that as well.