Run, run!!! The sky is falling!
I think there are people who actively seek out stuff to be alarmist about.
Like you, ragman, from all I've read I see absolutely no evidence of a drop in average intelligence scores at either end of the bell-curve. Autism, however, may be a different matter. As I tend to be anything but an alarmist (been accused of the exact opposite, in fact, not seeing obvious danger etc.), I quite agree that the most probable reason for an apparent increase in the number of children being diagnosed as autistic is simply better diagnostic measures.
But autism and intelligence don't correlate very well. We've all heard of what used to be called 'idiot savants,' a condition of people we now characterize as 'autistic savants.' If new statistical data were to show that the distribution of IQ scores is, indeed, showing a decline in native intelligence, that would say nothing whatever about autism.
Not so many years ago autism was lumped together with a whole host of other unrelated neurological maladies and patients were simply locked away in psychiatric wards. Thank goodness, we've come a long way since then. With the proper support, most autistic people can learn to function quite well within a structured social framework. And there really ain't that many of them.