More (I found this site linked to from the NIH):

ADHD has biological origins that aren't yet clearly understood. No single cause has been identified, but researchers are exploring a number of possible genetic and environmental links. Studies have shown that many kids with ADHD have a close relative who also has the disorder.

Although experts are unsure whether this is a cause of the disorder, they have found that certain areas of the brain are about 5% to 10% smaller in size and activity in kids with ADHD. Chemical changes in the brain also have been found.

Recent research also links smoking during pregnancy to later ADHD in a child. Other risk factors may include premature delivery, very low birth weight, and injuries to the brain at birth.

Some studies have even suggested a link between excessive early television watching and future attention problems. Parents should follow the American Academy of Pediatrics' (AAP) guidelines, which say that children under 2 years old should not have any "screen time" (TV, DVDs or videotapes, computers, or video games) and that kids 2 years and older should be limited to 1 to 2 hours per day, or less, of quality television programming.

Many people with ADHD also have other issues:

Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD)

At least 35% of kids with ADHD also have oppositional defiant disorder, which is characterized by stubbornness, outbursts of temper, and acts of defiance and rule breaking. Conduct disorder is similar but features more severe hostility and aggression. Kids who have conduct disorder are more likely to get in trouble with authority figures and, later, possibly with the law. Oppositional defiant disorder and conduct disorder are seen most commonly with the hyperactive and combined subtypes of ADHD.

Mood Disorders

About 18% of kids with ADHD, particularly the inattentive subtype, also experience depression. They may feel inadequate, isolated, frustrated by school failures and social problems, and have low self-esteem.

Anxiety Disorders

Anxiety disorders affect about 25% of kids with ADHD. Symptoms include excessive worry, fear, or panic, which can also lead to physical symptoms such as a racing heart, sweating, stomach pains, and diarrhea. Other forms of anxiety that can accompany ADHD are obsessive-compulsive disorder and Tourette syndrome, as well as motor or vocal tics (movements or sounds that are repeated over and over). A child who has symptoms of these other conditions should be evaluated by a specialist.

Learning Disabilities

About half of all kids with ADHD also have a specific learning disability. The most common learning problems are with reading (dyslexia) and handwriting. Although ADHD isn't categorized as a learning disability, its interference with concentration and attention can make it even more difficult for a child to perform well in school.

Many people who suffer from one (or more of these) also do things like self-medicate - they tend to drink more, smoke more, do drugs, engage in risky sex, etc.

It's not just about "kids with too much energy" - it's an actual neurological disorder.  I don't think there's actually "more" kids with ADHD (although there may be, as environmental factors may contribute to it), but 40 or 50 years ago, many of these same kids would have just been labeled as "bad kids" and no one would have looked further into their issues.  In fact:

To be considered for a diagnosis of ADHD:

    * a child must display behaviors from one of the three subtypes before age 7
    * these behaviors must be more severe than in other kids the same age
    * the behaviors must last for at least 6 months
    * the behaviors must occur in and negatively affect at least two areas of a child's life (such as school, home, day-care settings, or friendships)

The behaviors must also not only be linked to stress at home. Kids who have experienced a divorce, a move, an illness, a change in school, or other significant life event may suddenly begin to act out or become forgetful. To avoid a misdiagnosis, it's important to consider whether these factors played a role in the onset of symptoms

Medicine may not always be the answer, but for those that need it, they really need it.  Depriving them of it is akin to depriving a diabetic kid of insulin - it's that serious.  I know - my sister is severely ADHD, so I grew up with it, and at 35 years old, she is still struggling with it.