If you visit various online communities you may notice that some people refer to autism in one of two ways. As autism or as Autism. As a mother of someone with Autism, I tend to use Autism when speaking of her and other people with Autism specifically. Autism is not a descriptive term, you see. It's not just a way to refer to a disorder that someone has.
Someone with Autism is part of a larger community, and out of respect for that community I prefer to capitalize Autism. It's a sign of respect for my daughter.
It's also really shorthand for Autism Spectrum Disorder, which would be capitalized anyway when being referred to in writing. Our specific place on the spectrum is Asperger's Disorder, of course, as I've mentioned in the past. That name of course is capitalized because it's named after the doctor who realized that there was a section of the spectrum that was encompassed by a specific set of descriptors. Along those lines, for me anyway, Autism isn't simply "a disorder, lowercase" but a "More Broadly Named Disorder, Capitalized."
I may be grammatically incorrect, of course, which might stick in the craw of grammar nerds. I understand. I really do. I'm a grammar nerd myself although I make allowances for conversational blogging and Facebooking (as long as Facebooking isn't treated as if grammar doesn't exist at all and those reading deserve nothing but incoherent text-speak).
Mainly, using Autism Capitalized is, for me, a recognition that there is a community of people that includes the people who have Autism and their caretakers, their therapists, their friends, family, and teachers.