Let me start by saying that I have High-Functioning Autism myself, with the advantage of remembering what it was like to be a child giving me insight into how autistic kids process.
That said, I never went to a Disney theme park as a child nor was I diagnosed as a child. I wish I had been to a Disney theme park as a child, as I'd probably be of more help now had that been the case. But I have been to Disney World with nieces on the spectrum, who absolutely loved almost everything about the trips, and I have other experiences with more young relatives on the spectrum.
My nieces did not need many accommodations. The extended family broke off into separate groups, giving me charge of the girls. Really, the only thing I can remember that was a "big" problem for the girls was the time that two girls decided to get as wet as possible in sprinklers outside Ariel's Grotto whilst the others were screaming because they did
not want to get wet. In the process, one almost took off all her clothes because babies had no clothes on (and that young lady was a teenager... reminded me of myself during my youth, taking off my swimsuit at a private pool party without realizing what I had done), and as I stopped her from doing so, I discovered she had soaked up the map (though that didn't matter, as maps were everywhere) and soaked up her father's cell phone.
The solution I came up with was to take them straight to the Mad Tea Party to dry off the phone. It worked!
Frankly speaking, the worst concern of mine is the rude strangers. A pain of a woman purposely plowed a baby stroller over my feet because I compared her speaking a foreign language to the struggles a niece was having to communicate to me. Why that comparison offended, I don't have the slightest clue. Communication struggles are communication struggles.
Keep a sense of unconventional humor and don't lose your head. Most importantly, put yourself in their shoes. I've got a nephew whose parents do not think should ever travel because of the severity of his autism, and I think that those parents are overly paranoid. Nephew trusts me and my husband. We would love to take him to
Disneyland or Disney World alongside our own children. Our own children are three infants and a five year old. They've not yet gone to WDW, though we plan on taking them between 2010 and 2012. Ideally, we'd like to experience the park before going on the maiden voyage of either the Disney Dreams Cruise Ship or the Disney Fantasy Cruise Ship. We'll cross that bridge in January 2011.
Generally speaking, Disney is terrific at accommodations. They surpass all other theme parks I've experienced or heard of. Disney has got a Gluten Free Casein Free menu for everything. Definitely take your nieces, Original Poster. You won't regret it and you'll want to go again! Of course, I second (or third or fourth or fifth) the recommendations found here. Definitely take a midday break with each day, for your sake and for the kiddos!
To anyone reading this who has questions, I offer you up my assistance and advice. Just ask!
Very adaptable to your nieces, and what kind of day they're having. We always drive down, so we have a car at our disposal, which is also a big help. You really sound like you've got the basic philosophy of traveling with ASD kids covered. I hope you guys have a great trip!!
I want to second this advice!!! 
which was very upsetting for him. One other idea from the boards (don't remember where exactly) that I tweaked a bit and used was cards. Jonnie is verbal but he regresses when he gets upset, eigther his speech impediments increase or he stops speaking so I started worrying about whether he would talk to a CM if he got lost (a very scary possibility, but a possibility) so I made little business cards with his name "I am Autistic I may not be able to answer your qestions" and all of our cell phone numbers. We told Jonnie to carry them and if he coulden't find us to hand one to a CM and then he diden't have to talk if he was scared or upset we also put one in his backpack, one in his wallet and one in his jacket pocket. We had a great trip but I did notice that Jonnie acted less and less Neurotypical the longer we were there. By the last day he was stimming constantly I think all the crowds and stuff got to be a bit much but he did have fun and talks about it alot still. Hope you have a great trip!
