I know many people attribute the growth in autism to improved recognition - that is, we are now diagnosing all the autistics who've been there all along, previously unrecognised or wrongly diagnosed. This makes a lot of sense to those of us who previously flew under the radar and were only diagnosed in adulthood.
However, there's been a scientific study (sorry I can't remember where I saw it), which worked out that there has been a genuine growth in autism cases, over and above the growth attributable to improved diagnosis.
Personally, I believe there are still a lot of undiagnosed aspies out there (especially women), but there has also been a genuine growth in classic autism cases as well, perhaps related to environmental factors.