So what does the increase, and the current estimate from a recent CDC study of 1/150 children, mean? First, here's some text from the article (emphases mine) ...
Autism everywhereI'm willing to bet the increase has nothing to do with herbicides, pesticides, immunizations, mercury, gluten, etc (though if we were to find the prevalence is five times higher in China I might reduce the bet -- except it's not). We've not found any fruitful leads when exploring those domains -- despite a remarkably vacuous Discover magazine article that was enthusiastically received by the Autism Society of America. (Kudos, by the way, to Mary Powell of the MN society for a sober response in this article.)
By David Peterson, Star Tribune
Last update: April 09, 2007 – 11:55 PM
... The number of kids classed as autistic is exploding. A recent study by the federal Centers for Disease Control and Prevention reported that autism is found in one in 150 children -- and researchers involved in the study say that may be an understatement. In Minnesota schools, the state's Department of Education reports, the number of students identified as autistic jumped from fewer than 1,000 students a decade ago to nearly 10,000 today.
In months, three committees of the Minnesota Legislature will hold hearings to explore the reasons why. "Special ed costs are exploding," said Roseville DFL Rep. Mindy Greiling, who chairs the K-12 division of the Minnesota House Finance Committee. "People who know a lot about it tell us that it's not just that we're getting better at identifying autism, there is actually more and more autism," she said.
"Herbicides, pesticides, various pollutants can cause brain disorders in fetuses and babies, and we want to look at that, not only to save costs but save the human costs of these tragedies," Greiling said...
... Nationally, advocacy groups are expressing alarm, using the words crisis, epidemic, even national emergency to describe the increase. In Minnesota, however, advocates are more restrained. Mary Powell, executive director of the Autism Society of Minnesota, speaks of "dramatic" increases in caseloads but hastens to note that that's partly because the definition is widening.
A debate simmers over what's behind the increase. Some people blame vaccinations though extensive medical studies haven't found a link, according to MayoClinic.com. Others, like Greiling, point to environmental influences...
The Combating Autism Act of 2006, which President Bush signed in December, authorizes nearly $1 billion over the next five years to combat autism through research, screening, early detection and early intervention.
Autism intervention is costly. According to the United States Government Office of Accountability, programs for a school-aged child with autism costs $18,800 per year compared with $12,500 for average special education per pupil expenditures.
"We struggle almost every day for resources for kids," said Dr. Dan McLellan, a pediatrician specializing in child development at Children's Hospitals and Clinics of Minnesota. The challenge for school districts is significant, said Anne Harrington, autism resource specialist for the Minneapolis schools. "We're scrambling to keep up with the needs."
Insurance companies, said McLellan, don't always help. "One big problem is that many decline coverage because contracts exclude 'developmental delays,' " he said. "We see it as a disorder. Some health plans still see it as a delay that schools can take care of."
A spokeswoman for the insurance industry said that varies according to the benefits package. "There is often a discussion of where the medical component leaves off and where the educational arena might begin," said Susan Pisano, vice president of communications for the Washington, D.C., trade group America's Health Insurance Plans.
... Researchers emphasize that they were prevented from using key data sources that would likely have pushed the ratio higher...
So what's going on? There's a reason I emphasized one line in the article as bold and red. Diagnostic labels drive resources. Autism, like all disorders of the mind, lacks the insurance coverage given, say, cerebral palsy (a disorder of the brain with physically disabling manifestations). On the other hand, the label of autism delivers enhanced educational resources, whereas low IQ does not.
8% of the population has cognitive abilities in the bottom 8% of the population . That's a rate of 12 in 150. If half these children get relabeled as autism the prevalence will reach 6/150, or 600% higher than it is now, before the trend levels off.
Why would children in the bottom 8% of the cognitive curve get relabeled as autistic, Asperger's, or "autism-spectrum disorder"? Because otherwise they won't get the educational resources they need and deserve. The definition of autism and autism-spectrum disorders is very problematic, and given the latitude to assign the diagnosis parents, teachers and clinicians will rightly do what's in the best interests of the child.
Consider this article from April of 2006 ...
Respectful Insolence: Evidence against an "autism epidemic"Shattuck has the data, but he's missed completely the driver. It's not "social stigma" (though autism, and especially Asperger's, is a much less depressing term than "mental retardation"), it's that most school systems don't deliver enhanced educational and social resources to children who are on the low end of the cognitive curve.
... Shattuck analyzes special education figures that are being used to bolster claims of an autism "epidemic" and finds them wanting. In essence, diagnostic substitution can explain nearly all of the apparent increase of autism as recorded by the number of children receiving special education services.
...Dr. Shattuck starts with an example from a different condition, mental retardation as one of his reasons for suspecting diagnostic substitution as a cause of the perceived "epidemic":...Second, prior research has established a precedent of diagnostic substitution in special education enrollment. From 1976 to 1992 the number of children in the mental retardation (MR) category decreased by 41%, whereas the number in the learning disabilities (LD) category increased 198%. There is considerable evidence that suggests this was because of a growing likelihood that schools would use the LD label for children with mild MR, presumably because a label of LD was increasingly seen as carrying less stigma than MR. Finally, a recent epidemiological study depicted a downward deflection in the incidence trend of other developmental disorders just as the trend for autism made a sharp upturn in the early 1990s, again suggesting the possibility of diagnostic substitution.
So, obviously, I think a lot of the growth in the diagnosis of autism-spectrum disorders is being driven by diagnostic substitution. I think the substitution is driven by a just and honorable desire to provide children with the educational resources they need in an increasingly complex, competitive, uncertain and demanding world. The substitution can occur without the necessity of deception because we don't have good models to categorize disorders of behavior and cognition.
BUT, is there anything else going on? Is there a "real" change in either the prevalence, or the nature, of cognitive disorders in children? What would we find if we compared the "bottom"  8% of 1950 with the "bottom" of 2007? Would the minds measure out differently? My wife thinks there's a difference, and she's often right about such things. Old-time clinicians think their young patients look different. Does some of the "diagnostic substitution" reflect an underlying change in the nature of cognitive disability? Is there a real rise in the rate of disability?
Those are biologically interesting questions. I would not be surprised if there have been some real changes, but more on the level of a 50% increase or change rather than a 1000% increase. That's still, by the way we measure things, a pretty significant increase over a period of 10 years.
I suspect differential mating (like-marries-like) plays a role , and one can always speculate about environmental effects (ultrasound and neuronal migration?). If I had to really, really, speculate however, I'd wonder about an infectious disease process  ...
 I know it's a tautology. It's also important.
 It's important to remember that what puts one at the "bottom" of today's world might play out differently in another world. I suspect that for most of human history my eagle-eyed, hyper-alert, hyper visual processor son would have been supporting me rather than vice-versa.
 This, by the way, is probably how the human mind evolves. This should vary by country and culture so it's a testable hypothesis.
 Not to be confused with an immunization side-effect
 I made up the 15 years number, but eyeballing the curve that seems about right for an exponential-seeming process that increases 10 fold in 10 years.