From an article online:

There is very little that frustrates me more than a well-intentioned yet ignorant conclusion that ADD and ADHD are nothing more than a lack of discipline. Don't get me wrong; the people who hold this viewpoint aren't usually where my ire is directed. I hate the insidious and even dangerous nature of the opinion itself. Granted, there are cases in which a child is diagnosed with ADHD who is merely active or undisciplined. However, there are also many cases that go completely undiagnosed because people are uninformed or misinformed about the disorder. (Though I hate to use the negative label of "disorder", much less "disease", as I feel that both are misleading.)

I agree with the opinion that many parents seem to be afraid or otherwise unwilling to discipline their children. The widespread result is parents who would rather be buddies than authority figures raising children who have no respect for other people. People seem to equate firm discipline with spankings or abuse, when they are actually unrelated. Discipline encompasses positive encouragement for good behavior as well as consequences for unacceptable behavior.

Children with ADD differ from children with discipline problems in that punishment generally has no helpful effect on those with ADD. These children aren't able to focus any better when faced with punishment. There may be marginal improvement for a brief period while the child desperately tries to live up to expectations and earn approval, but the old pattern doesn't take long to reassert itself.

I speak from experience that disciplining a child for problems caused by ADD can cause depression. As a girl with ADD, I went undiagnosed. (This is a common problem only recently being researched and rectified, but that's another article in itself.) Many girls with ADD do not show symptoms of the more widely recognized hyperactivity. Instead, they might be compulsive daydreamers like I was, unable to keep their brains focused on the task at hand. My homework (among other things) suffered as a result and I was frequently being reprimanded by my parents, my teachers, and other authority figures in my life.

The reason this is so harmful to a child with ADD is that, aside from having a limited attention span, many ADD children are very sensitive to criticism. Disapproval holds much more weight with these children than with many others. This doesn't mean that these children shouldn't be disciplined at all; it only means that a little will go a very long way withthem. On the other side of the equation is that a lot of discipline for something the child isn't able to control can lead to other problems, such as low self-esteem.

There are people who tend to scoff at the idea of self-esteem, as well. I admit that it's become a modern catchphrase, but that doesn't make it any less important. A well-adjusted child, with or without ADD, is better equipped to handle the problems and challenges of life. A child with ADD who is punished for something ADD-related or told that "it's just a matter of trying harder" will become frustrated with the inevitable failure to meet others' standards. They might become angry and bitter toward other people or, as in my case, with themselves. This sets them up for even more failure as they come to believe that those problems and challenges of life are insurmountable. It may even keep them from doing the things they enjoy because they see themselves as incompetent or even unworthy.
This couldn't be further from the truth. Children (and adults) with ADD have many positive traits and are sometimes better suited to certain tasks or professions than many people without ADD. A great deal of the general public are unaware, for instance, that people with ADD tend to be very creative, very personable, and very good at multi-tasking, to name just a few things. Instead of drawing false conclusions about ADD, we should ask questions and do our best to be better informed. This solves two problems; not only would genuine ADD be recognized and effectively treated (not necessarily with medications - but that's yet another article) but we would be better able to know the difference between children with ADD and children who are just undisciplined. And knowing, as G.I. Joe so wisely told us, is half the battle.