On our first visit we were very afraid that Ben would be overwhelmed, but for him at least it turned out not to be an issue. As soon as he walked onto Main Street he knew exactly where he was, and was deleriously happy. That being said, we definitely had some coping tools in place.
First off, we are always sure to bring along his CD player and headphones. If the crowds get too much for him, he can always put those on and screen out the distractions.
Second, be sure to stop by any Guest Services location and pick up a Guest Assistance Pass. When you explain the childs issues, they will be more than happy to give you a pass that lets you bypass most of the lines. In most cases you go in through the FastPass entrance, or failing that you go in through the wheelchair entrance. It's not quite a front-of-line pass, but it at least moves you from the crowded, noisy lines and into a faster and generally quieter queue.
Third, and most important, don't push it. Know your child's limits and don't try to push them past them (unless they are actually primed to be pushed a little). If you go into the park, do two things, and the child is ready to go, then be preparted to take a break. Even on the most crowded days you can find quiet spots to decompress for a while. Never be afraid or ashamed to ask a Cast Member for assistance if you need it. In my experience, the Cast Members are almost always willing and eager to go above and beyond to help your child enjoy the visit.
I seem to remember another website that had some specific tips on taking autistic children to WDW, but the exact one is slipping my mind at the moment. If I remember it I will come back and post a link.