I've worked with a lot of children with Autism at the therapeutic riding center I work at. I've seen wonderful results from riding for many of these children. Because Autism often comes with some sensory integration issues, the input from the horse can really help settle the riders. Also, the reciprocal relationship with the horse can be wonderful for their social skills, as a lot of children are able to relate to the horse in ways they can't relate to other people.
I'm not a huge fan of "The Horse Boy" that tinyliny posted because the father said that hippotherapy "cured" his son of Autism. It is, however, a great homage to the benefits of riding. I find work by Temple Grandin to be more impressive, especially because it's an inside view to how riding helped her.