I think we've gotten better at diagnosing mental illness. Kids who would have been diagnosed as generally mentally retarted a generation ago are now given more specific diagnoses, which facilitates treatment.
I doubt there are any more actual cases of autism, we're just getting better at catching them.
I think we've gotten better at making up new mental illnesses to suit modern cultural trends. Much of the "autism-spectrum" illnesses are nothing more than a new way of describing kids in med-speak who used to be just weird, odd, ill-mannered, or unsociable. Things that didn't need to be "treated" just dealt with.
I was in a bookstore recently and I looked a book that was intended to be a kids guide to understanding autistic kids. What I found out was that apparently I'm autistic because I only like to talk about a limited number of things, I can be highly focused on tasks, I don't like people touching me without permission, and I don't like good experiences with friends to end. Outside of the smallish percentage of people who literally are non-functional, much of the "autism-spectrum" disorder is failure to be the All-American Kid.