Salon has an interesting article about spanking kids today. (If you don't subscribe, you'll have to sit through an ad to read the article.)
I don't have kids, but I have spent time thinking about raising them when I do have them. The issue of whether it's ever OK or desirable to spank kids is a tricky one. Most of us were spanked as kids to some extent, and I think parents who spank but are responsible, kind, and attentive are better than non-spanking parents who aren't all there in other ways. In other words, I don't think spanking is always materially harmful or that it is the main difference between good parenting and bad.
Ultimately, I decided that spanking kids is not a good idea for me for a pretty simple reason: I do not have that much self-control. If I thought it was acceptable to hit my child, then when the kid really pissed me off (as they tend to), I would probably be abusive. I don't hit my coworkers, friends, or lovers because it's clear to me that it's really not OK to do that, at all. By having the same standards about children, I can rule out physically abusive behaviors. (Maybe I am wrong about myself, and I would never lose my temper enough to beat my child, but why risk it?)
I don't think I'm alone in this, and I think advocates of spanking are doing families a disservice for that reason. By giving parents permission to hit misbehaving children, they are increasing the likelihook of an abusive punishment. (This assumes that spanking is not always abusive, which is an arguable point.)
If you lose your cool and yell at your kid (especially if you refrain from saying anything really hateful), you can apologize later. If you issue disproportionate punishments ("you're grounded for a year!"), you can retract them later. But if you beat the crap out of your kid, there is no taking back your child's traumatic experience.