the term hippotherapy means "treatment with the help of the horse". There is some good research showing benefits and a great deal of anecdotal evidence. NARHA North American Riding For the Handicapped Association and CANTRA Canadian Therapeutic Riding Association are good resources.
Once you get involved in this type of therapy, you get hooked. Kids and adults alike love it and love the horses. There are many physical benefits, but also social and emotional. There is an excellent book by Jan Spink called Developmental Riding Therapy.
NARHA have good articles on their website and many references.