From the comments of everyone on this thread, it seems there are two groups: A) Those who were not just spanked, but rather beaten by their parent(s), or who otherwise suffered unreasonable administration of spankings, whippings, etc., and who would never lift a hand against their children today, and B) Those who were spanked sparingly as children and who feel it was a reasonable form of discipline and administer it today.
In other words, it seems that whether you spank your children or not, and in what fashion, depends almost solely on the circumstances in which you were spanked as a child. That might sound pretty straight-forward, but it's actually rather interesting, because, if true, it means we don't come to these conclusions whether to spank or not based on what we know of child psychology and development, but rather, our own emotions. We might find information to back our reasons for spanking or not spanking, but it's probably an effort to support a belief we already hold.
If I may posit a theory here without denigrating the experiences of those who were physucaly abused, I'd say that those who don't spank their children do so because they do not want to place themselves in the same position as their abusive parent, even if it were a matter of mild, sparing spankings, it raises too many painful emotions.