If you discipline your first child well enough, the mere threat of punishment will work on his/her younger siblings. For instance, my parents used to tell my little sisters that if they misbehaved, they'd give them a white napkin, a wooden block, and a knife, and force them to slice off a joint of their pinky finger themselves, wrap it in the napkin, and present to my dad as a gift. Apparently, they never really took the threat seriously, and they just laughed it off, until one day the older one got in so much trouble that my parents decided to give her the shock of her life. They sat her down on the couch, wheeled a large box from their closet into the living room on a dolly, and released me from my Skinner box to meet them for the first time. Once they saw my hands, they never disobeyed my parents again. Today, I'm a free man, but I type with my knuckles, and I'm not a fan of authoritarian parenting. I guess I turned out alright, though.
Seriously though, this is an interesting discussion. My own experience tells me that spankings are pretty useless for the most part, but I'm a pretty unusual case...I was spanked a few times when I was a toddler for silly offenses, but I never was spanked as punishment after that. I doubt the spankings had the intended effect anyway, because I couldn't have possibly done anything all that dangerous or morally wrong anyway that I should have known better about (the worst thing I did that earned a spanking was probably when I ruined and almost entirely unravelled one of my favorite socks during naptime, trying to get rid of a loose string, and then blamed it on my imaginary friend

). I was also hit a couple times or so in a flash of anger for no particularly good reason, but that obviously wasn't discipline, so that doesn't count. Throughout most of my early childhood, time-outs were the main method of punishment. I can remember sitting in time-out and tracing the designs on the chair, but I can't really remember what I ever did to get it...odds are I was probably just getting out of hand and needed to cool down or something. Later on in my childhood, groundings were the main method of punishment. Those didn't really work all that well, though...even though I was a pretty good kid and adults always loved me, I was stubborn as hell, and the one and only way to make me change my behavior was to convincingly explain to me why my behavior was wrong, hurtful, etc. The problem for my parents (my dad especially) was that they could rarely ever put forth a convincing argument, because the "worst" of my behavior was pretty much talking back and arguing about the unfairness of undeserved groundings (which generally followed every report card and parent-teacher conference, since I always got in trouble for talking in class and trying to make friends...for about six years straight

). Actually, my mom even feels bad about all of the groundings now.
The point of all this: In my case, "discipline" of any kind didn't contribute much whatsoever to my manners or morals...although it certainly helped me hone my rebellious side, because I fought tooth and nail against every minor and petty injustice I was dealt, no matter how much worse the consequences would be because of it.
Rather than discipline, I think there was one and only one thing that instilled a moral code and sense of decency in me: Many of the most important adults in my life took the time out of their everyday lives to teach me and help me understand how to be a good person and what it means, and my mom in particular always valued that as a trait above pretty much all else. My mom and other family members always had very high expectations of me, and they made sure I knew what they valued in people. Combined with them constantly stating their high opinion of me, I think I developed along the path I did in the initial stages because I didn't want to let them down.
I like a lot of people's posts in this thread, especially Freedom 4 all's, wizardwatson's, conza's, and Original Intent's. I'm not a parent yet, so my views might not "count," but here's my perspective:
Discipline works to make sure your children understand troglodyte-inspired concepts like authority, punishment, and reward, but that alone will only bring them up to stage one or so of Kohlberg's stages of moral development.

Discipline focuses on the behavior, and it can effectively prevent poor or hurtful behavior on a temporary or even a permanent basis, but it's nowhere near enough to give a child a meaningful understanding of morality or what kind of person they should strive to be. Even though most adults behave well enough to successfully function in society and play nice with others, I feel as though very few are particularly "good people" at heart. Sure, most people are capable of love and empathy to a greater or lesser extent, but so few ever really move beyond the shallow morality of children. I don't just want my kids to grow up to be merely functional and superficially moral enough to avoid public displays of flagrant asshattery; I want them to grow up to be exceptionally moral. It's important to me that they grow up with a universal sense of empathy, a strong personal sense of justice, and firm principles. I want them to understand that doing the right thing just because it's the right thing is not only an end in and of itself, it's the single most important concept they should base their lives around.
Obviously, discipline alone isn't going to achieve all that, but I'm sure it will become necessary to get things under control on occasion and to stress consequences for actions. I don't foresee any spankings though, unless my kids do something that's dangerous to themselves or to others (as someone else said, if they ever do something that absolutely must not be tolerated under any circumstances, I'd have no choice but to send an immediate message that I mean serious business). On the whole, I don't see how they're any more effective at stopping/preventing poor behavior than using other kinds of discipline or deterrents, and they certainly don't teach any kind of moral lesson...especially if they're done in anger, which
really sends the wrong message. Making spankings illegal may be downright insanity, but it still doesn't mean any kind of physical punishment is a particularly good idea. I know a whole family of quite successful people who "turned out just fine" by all appearances after an upbringing filled with corporal punishment and "strong discipline," but beneath the surface, at least half of them are shockingly cruel, abusive, and evil people. Besides, spankings wouldn't be very consistent with everything I want to teach my kids anyway, and I'm sure they'd eventually call me on it.

When it's necessary to discipline them for acting out of line, I'll probably opt for rescinding privileges and putting them in time-out to make them cool off. If they commit some particular offense against another person (like hitting each other, biting each other, etc.), I'm sure I can come up with some creative form of justice that will force them to make full amends with their victim.
Bottom line: Discipline affects behavior, but to make your child into a good person, I think you need to work on a deeper level, and not just following some "incident" either. When discipline does become necessary though, I don't really see the point of spankings other than proving your physical dominance and ruling by fear, and any kind of corporal punishment might do more harm than good.