Diagnosis of Autism versus Mental Retardation

I was reading a news article a few days ago about the substantial increase in the diagnosed cases of autism in the U.S., and how people were attributing it to mercury contamination in fish, vaccinations, etc. This made me curious and I started looking at some figures and their supporting articles. Now, I will state for the record that I have friends that have worked tirelessly with autistic children, and it is both heart-wrenching and rewarding at the same time. However, with modern medicine, I found it hard to believe that autism was spiking so dramatically, as shown below.



Then I found a plausible explanation for the spike. One set of researchers suggested that for the rise in autism over the past 10 years (I believe roughly 9.3 more cases per 10,000), there was a roughly equal decline in the diagnosed cases of mental retardation (down about 9.1 per 10,000). There is some overlap of around 30% of autistic children that are also mentally retarded, but it seems like an ongoing change in the diagnosis of children's developmental problems seems somewhat more likely than an autism epidemic. I would love if anybody out there has any more information (I know that I will keep looking as well).