After telling people that I am a researcher studying autism, I am often asked the above question. Unfortunately, there is no straight answer for this question. If one goes back in the literature, it is true that for decades (several studies conducted from 1966-late 1990's) the prevalence estimates were on the order of 4-5:10,000 while studies conducted over the past decade have estimated the prevalence on the order of 2-6:1,000 with one study showing as high as 12:1,000. So, why this difference? Is it a real increase in the prevalence of the disorder or something else?

My opinion, and I stress this is only an opinion is that it is likely a combination of several factors in no specific order.

1. We are certainly more aware of the specific deficits defining ASD now as compared to 10 or 20 years ago. Children who previously may have been given a general "Mental Retardation" diagnosis (on the lower end of the spectrum) or just considered socially awkward and not given a diagnosis are now recognized and being correctly diagnosed as having autism and Asperger's Syndrome.
2. The diagnostic criteria for Autism and Autism Spectrum Disorders have changed over the years with Asperger's Disorder only being included in the DSM in 1994.
3. There is now less of a stigma to being diagnosed with autism. In fact if the child has a diagnosis, they are likely to receive extra services from the state including educational services and therapy. Given this change in the stigmatization and opportunity for benefits, parents are perhaps more likely to actively request a diagnosis.
4. Children growing up now are exposed to many more environmental stimulation than children in the previous decades. The influence of the environment on neural development is clear and changes in the environment are much more likely to cause changes in prevalence than changes in genetic mutation over the course of several years.
5. ...?

So, the answer is, yes there seems to be an increase in prevalence over the past several years. But the cause of this apparent increase is still unclear. Whatever the cause, if approximately 24,000 children (just in the U.S. alone) are being diagnosed with a disorder, this is clearly a major public health concern that needs to be studied in an effort to understand the disorder, establish the necessary services for both the children and adults with ASD, and hopefully curtail the current epidemic.