That depends on how you define "mental illness". If you define it as "something in the DSM-IV, treated by a psychologist, with primarily mental symptoms", then yes, it's a mental illness. But consider: Under this definition, many other things are mental illnesses: Mental retardation, learning disabilities like dyslexia, ADHD, speech impediments, Alzheimer's. Putting autism in that group makes sense--even if "mental illness" has connotations that those things don't have.
On the other hand, let's define mental illness as "a condition with emotional, behavioral, and sometimes psychotic symptoms, treated by a psychologist, which causes distress and impairs function." Under that definition, none of the above (including autism) qualify. This is the more commonly used definition of mental illness...
You know, reading the above paragraph, I realize I have trouble properly explaining the average-person idea of "mental illness"; my definitions don't really seem to draw a mental line between the two groups. All I really have is a large list of examples which people would consider to be "mental illness" and I have trouble properly drawing out the common characteristics.
So maybe it would be better to just make a list of what most people would consider to be "mental illness".
- Various anxiety disorders: Panic attacks, GAD, OCD
- Schizophrenia and other conditions that involve psychosis
- Depression and related conditions
- Bipolar disorder and related conditions
- Hypochondria and other psychological conditions with physical symptoms ("It's all in your head")
- Dissociative identity disorder (multiple personalities)
- Eating disorders
I went through literally all of the DSM-IV categories to pick those out. (Wikipedia rocks, incidentally.) They made up barely a quarter of the available diagnoses. Also note that the previous list includes mostly very severe psychological disorders--the "flashy" kind they put on TV a lot--and the ones you're most likely to be hospitalized for. I actually had to debate whether to put hypochondriasis on that list, because most people wouldn't immediately associate it with "mental illness" so much as "making a fool out of yourself".
"But," some say, "Asperger's isn't a disability. If it isn't a disability, it can't be a mental illness." Hate to burst your bubble, but yes, Asperger's is a disability. That there are positive aspects to AS doesn't change that we lack some skills the world expects of us. By the social model, we are disabled, just like deaf people and wheelchair users who, just like us, simply need the right environment. Social model: A deaf person in a hearing world is quite disabled; a deaf person in a signing world is not disabled. An Aspie in a world where people use lots of vague communication is disabled; in a world where people say what they mean (and where special interests are OK and people don't create overwhelming sensory situations), he's not disabled. But of course, the social model of disability is quite another issue altogether.
The "Asperger's is not a mental illness" protest, I think, stems from this popular concept of mental illness as something that makes you "go crazy" and get put in the looney bin. This is an unfortunate state of affairs because, technically, in the medical sense, AS is a mental illness; but in the colloquial sense of the term, it's far from it.
How to explain Asperger's? Describe it as neurological--"My brain works differently". This should evoke the right connotations in the listener's mind.