The diagnosis of Attention Deficit Hyperactivity Disorder began around 1980. It was not the first time a diagnosis was given to hyperactivity in children; it is when the American Psychiatric Association adopted the name that is currently used. Symptoms of ADHD, such as hyperactivity, have been documented in medical science since the mid 1800s. Prior to 1980, ADHD was known by various names: Hyperkinetic Disorder of Childhood or Minimal Brain Dysfunction.
The American Psychiatric Association bases the current diagnostic criteria for ADHD on the Diagnostic and Statistical Manual. The diagnostic criteria for ADHD include detailed information on how and when a diagnosis should be made. It provides information on symptoms as well as three specific subtypes and the symptoms related to each.
In many physical illnesses, there is an absolute diagnosis. This involves blood or laboratory tests that can definitively point to a specific problem. Based on this, physicians are able to target and treat specific illnesses. ADHD, and other mental illnesses, do not have a blood test that provides a definitive diagnosis. It is instead based on observation, by parents, teachers and caregivers. Questionnaires regarding behaviors in different environments help a doctor determine if a child, or adult, has ADHD. Doctors look not only on the behaviors, but how long they have been present and whether the behaviors are excessive or not age appropriate.
ADHD, however, is a legitimate diagnosis and can be accurately diagnosed by qualified physicians. ADHD is recognized around the world. In January 2002, an international consortium of scientists signed a consensus statement, which states, "We fear that inaccurate stories rendering ADHD as a myth, fraud or benign condition may cause thousands of sufferers not to seek treatment for their disorder. It also leaves the public with a general sense that this disorder is not valid or real or consists of a rather trivial affliction. We have created this consensus statement on ADHD as a reference on the status of the scientific findings of this disorder, its validity, and its adverse impact on the lives of those diagnosed with the disorder..." This consensus was signed by hundreds of scientists and physicians around the world. 
Some experts believe that, not only is ADHD not overdiagnosed, it is actually underdiagnosed. In an article on MedicineNet.com, "Adult ADHD Underdiagnosed", David Baron, DO of Temple University, discusses the underdiagnosis of adult ADD, "Man other psychiatric disorders, such as anxiety and depression, but ADHD is more of a constant. Because of that, people don't label it, they dismiss it, they say, 'Oh, I've been this way since the second grade." I think adult ADHD is underdiagnosed."
Another article that appeared in Family Practice News, "ADHD Underdiagnosed in Girls, states, "Girls with the condition are dramatically underdiagnosed and untreated, In fact, we may be missing as many as 75% of girls with attention deficit disorder." 
So, while it is quite probable that some children without ADHD have been misdiagnosed, it is also quite probable that children with ADHD go untreated.
Some of the reasons some people point to overdiagnosis:
ADHD Understanding the Problem, Updated 2008, March 24, TelosNet
"About Us - Feingold Association", 2008, Feingold Association of the United States
 "Almost Half of Kids With ADHD Are Not Being Treated, Study Finds", 2006, Aug 6, Washington University School of Medicine
 "Is ADHD Being Overdiagnosed", 1998, Nov 16, Meg Kissinger, Journal Sentinel
"ADHD Controversy", Date Unknown, Guy, FDU
"Dramatic Rise in ADHD Sparks Controversy" 2000, Dec 15, Diane Weaver Dunne, Education World
"ADHD", Reviewed 2005, Jan 30, Ronald Pies, WebMD
"Myths and Misunderstandings", 2007, Author Unknown, Help for ADHD, National Resource Center for ADHD
 Report 10 of the Council on Science and Public Health: Attention Deficit Hyperactivity Disorder, 2008, June 2, CSAPH, American Medical Association
 "What Causes ADHD?", Last reviewed 2008, June 26, National Institute of Mental Health
 "International Consensus Statement on ADHD, 2002, Jan, Clinical Child and Family Psychology Review
"Adult ADHD Underdiagnosed", 2008, May 6, Charlene Laino, MedicineNet.com
 "ADHD Underdiagnosed in Girls", 2000, April 1, Ellen B. Littman, Family Practice News