It's so not a mental illness, that it's being phased out from the DSM in the DSM-V.  They're eliminating it and inventing a whole new category of "mental condition" called "Autism spectrum disorder", lumping "aspies" and profound autist children in a single category.

Psychology / psychiatry are by and large not sciences.  They are not evidence based.

Note that I'm not saying "autistic children have nothing wrong with them" above.