The New York Times published a story on travelling with autism (see here). It says ...

Parents carry a letter from their child’s doctor explaining the condition (to whip out at the airport or to present at guest relations at Disney World, where it can secure a pass to circumvent long lines). They take along familiar toys and a DVD player so favorite movies can be watched en route. And if the child is on a gluten-free, casein-free diet (thought to relieve allergies and other medical ailments that might be distracting to someone with autism), they cart along a lot of food, too.

There are some other tips on the internet, for example http://http//autism.about.com/od/copingwithautism/ht/vacationstep.htm