I worked in a special ed preschool in a public school for awhile. We had one class exclusively for autistic children. ALL of those children were textbook, classic autistic.
In my classes (morning and afternoon), we had one child who was classically autistic whose parents didn't want him in the other program, 1 child with Down's Syndrome, 1 child with extreme physical and mental handicaps, about 5 kids who had something going on because normal discipline, firmness and consistency were ineffective, and maybe 8 kids who needed speech therapy.
The other 24 kids were all diagnosed with something (had to, in order to qualify for the program) but they seemed pretty normal to me.
There is a LOT of diagnosing going on, but from what I've seen, the autism diagnoses have been spot on.
I'm currently a one on one aide for a third grader with Autism. He's been at our school since he was in kindergarten and there is no misdiagnosis with him.
He is very high functioning but without the help of medication this boy would be a danger to himself and to others.
It should also be mentioned that there is an Autism spectrum scale, with mild Asperger's on one end and non-verbal, low functioning Autism at the other end. It's much more complicated than I ever knew before I started working in special education.