I have 3 children and I spank them when they CHOOSE it. My reasoning, passed down to me from my parents:
1. Disobedience is the ultimate no-no, if you choose it, you choose a spanking.
2. A spanking is not my choice, it's yours
3. I don't like spanking you but I have to do this, or I'd be a liar since I told you I would.
4. I love you. And that means I love you enough to teach you how to be a respectful, responsible person...even if it hurts me and you a little bit in the process.
That being said, I think that spanking has it's upper age limit. There are far worse things that I can do as discipline for my 9 year old...like taking away the playstation, laptop, gameboy advance. Spanking is never done to injure, but as a physical reminder of actions and consequences...like when you stick your hand in a fire. There are no tricks; the consequences have already been explained, and the line has been drawn. We never spank out of anger, and always make sure to explain why our kids are getting the spanking...mainly that it was their choice.