I wanted to reply and encourage anyone with an autistic child to visit WDW!! I wrote a lengthy trip report about our family's first visit to WDW in August 2001 and it is posted on Brian Bennett's mouseplanet website. I'm currently working on the report about our second trip, in January 2002, to ASMu (great resort!). My 7 year old son has PDD-autism spectrum disorder and my desire for him to experience WDW was greater than my fear of him "melting down" because of the noise and crowds. His development literally leaped a year after the first visit and another year after our second visit in January 2002. Travel to WDW now has a high priority in our budget.
I strongly encourage ANYONE with a disabled child to request and USE the Guest Assistance Card. There should be no "moral dilemma" in my opinion -- your child is at a disadvantage every day of his life, so if the GAC makes his life easier (and yours) at WDW, then use it! We imposed limits on ourselves, i.e. we never used the card for posted waits of 20 minutes or less and never twice for the same ride in the same day. 90% of the CMs were gracious about it and we didn't get ANY glares from people because they thought we had a fast pass! No big deal.
I discovered the joys of WDW late in life (first visit at 41) and I feel it is the ideal place to vacation with a special needs child. No where else will you get the same level of safety, cleanliness, support and variety as at WDW. Let me join the crowd and advise a midday break at your hotel pool prior to returning to the parks. If your child, like mine, is terrified of the noise of fireworks yet loves them from afar, get foam earplugs. With those, and a hooded sweatshirt tied tight (or my husband's big hands clamped over his ears), my son was able to enjoy Illuminations and so were we!
Our next visit is in November and we're booked at ASMo - I know my son will be thrilled with the theming there.