There is strong evidence that either the number of cases of autism has increased dramatically or else the severity of existing (static?) caseload has increased. There are no clearcut answers to this; the few studies conducted in deacdes past are sorely lacking in both size and scope, and the more recent studies which correct these defficiencies have nothing to base a time-trend analysis.
Some experts believe empirically in the theory of an “explosion” of caseload based upon limited evidence, with myriad pet theories as to why. Others argue that it is merely better recognition and expanded diagnostic criteria, although they can offer little in the way of sound evidence to back up their opinions. Even amongst parents and persons on Spectrum their is little agreement, and we have seen of late a growing “us vs. them” mentality developing as adults on Spectrum intimate that parents are misguidedly treating the autistic population as incapable of having an opinion while parents take their own experiences and try to see how this can relate to persons they communicate with on Spectrum who are high-functioning to say the least. (Brief aside: many of the most vocal Auties and Aspies claiming to speak for all on Spectrum are in truth sef-diagnosed, and some have hinted that perhaps they do indeed have some form of personality disorder, but autism is likely not what they have.)
Here is a paper written by Mark Blaxill et al, a founding member of SAFEMINDS which looks at this question in a way that previously published attempts have not. I do not think this will by any means settle the matter, but it just may point the way to the structuring of studies which will.
http://download.journals.elsevierhealth.com/pdfs/journals/0033-3549/PIIS0033354904001347.pdf