September Morn,
With all due respect, "A Beautiful Mind" is about a Nobel mathmetician with schizophrenia (not autism). Just watched it a couple weeks ago. Wonderful. The first part of the movie is filmed from the perspective of the man who is ill--really helps one to understand that point of view.

Someone mentioned "Rain Man" which is about a man with autism, which is not a mental illness but a neurological disorder.

"As Good As It Gets" is great about a man with OCD, but the OCD is just a part of him--it's not the main point of the movie. Love it.

"Nell" isn't about mental illness per se, but does deal with some pretty significant childhood trauma and communication disorder. Jodi Foster starred, with Liam Neeson.

A foreign movie comes to mind from the 80's -- French maybe? "Blue" about a man with a girlfriend who deals with some pretty serious issues, undiagnosed but looks like could be bi-polar disorder. Powerful and realistic.

"Leaving Las Vegas" is about alcoholism and depression. It's pretty depressing to watch.