Do you have any evidence medications-- especially in the case of ADHD-- delay development? I was under the impression that what evidence we had (under longitudinal studies) showed a slightly increased chance of developmental delays going away by adulthood than in people who took no medications at all. (ADHD itself involves developmental delays, remember.) That could be a false claim by drug companies, but this isn't the kind of thing it would be hard to prove, and my sources for that info were university researchers. The downside of meds seems to be in their side effects and potential addictiveness.

I highly doubt there is a serious problem with overdiagnosis. I think it's more theproblem that meds really shouldn't be the first choice of how to treat ADHD (as with depression and other illnesses) unless a person is expected not to respond to other interventions. The reason it goes first is because psychological care is lengthy and expensive, and adjusting a child's environment simply isn't practical in most cases.