What do you attribute the huge increase in Autism diagnosis to?

According to several studies I have read recently, there was an 800% increase in the positive diagnosis of Autism in the decade 1993 to 2003.
I have my own theories on this, right or wrong. I am interested to hear your opinion on the matter.
Do you think it is purely to do with the improvement of the diagnostic methods, or is there some suspicious underlying reason for the huge increase?