One to two people per 1,000 has autism, and about six per 1,000 have an Autism Spectrum Disorder (Newschaffer CJ, Croen LA, Daniels J et al. (2007). "The epidemiology of autism spectrum disorders". Annu Rev Public Health 28: 235–58.). Not all with autism or an autism spectrum disorder are necessarily disabled by their condition. Some are able to function while others find that their condition substantially limits that person's ability to perform activities of daily living that most others take for granted.
There is a mistaken belief that people with Autism have issues feeling emotions. Autism was once considered a type of mental illness, but it is now recognized as a sensory processing disorder. People with Autism Spectrum Disorders may experience difficulty recognizing and processing subtle social cues in facial expression, body language, inflection, and intonation which results in confusion in learning how to recognize and exhibit expressions of emotions, but not the feelings of those emotions.
Other sensory processing disorders include blindness (vision processing) and deafness (auditory processing). Service dogs can be trained for some people with Autism to help them gain independence, confidence, and the ability to perform activities of daily living that they could not otherwise perform. For the most part these dogs are trained to perform tasks similar to those of service dogs for other sensory processing disabilities. A guide dog for a person who is blind signals the handler when the team approaches an intersection so that the handler knows to stop and check for traffic. An Autism dog might be trained to do the exact same task, except that instead of giving visual information ("I see an intersection"), the dog gives prioritizing information ("I recognize a situation that requires focused processing").
An Autism service dog might signal the handler of important sounds, like that of a smoke alarm. When a person is trying to process 20 different things, including the sounds of crickets, a smoke alarm, the smell of the fabric softener on the sheets, the feel of the fabric on his or her skin, and so on, it may take that person a while to get down the list to the really important information: the smoke alarm. Those without processing impairments automatically recognize the urgency of the smoke alarm, but many with Autism cannot do so without careful consideration. They certainly know what it means and that it is urgent, but they must think it through step-by-step to arrive at the conclusion that a speedy exit is required. As with a person who is deaf, a trained service dog can signal the person with Autism of an important event, such as a smoke alarm, the phone ringing, someone at the door, the alarm clock, the kitchen timer, the baby crying, etc. The dog's signal to the handler reminds the handler to drop all other processing and focus on the sound being indicated by the dog.
Autism service dogs may guide a confused handler from an overstimulating situation on command, just as a guide dog would guide his or her handler home on command. Alternately, the dog might be trained to find a specific person, perhaps a caregiver, when the handler is over-stimulated.
Autism service dogs can assist with notifying their handlers when the handlers are doing certain repetitive behaviors, called "stimming." Often these behaviors, such as hand flapping, are beneficial to the person by being calming. Some stimming behaviors, such as head banging, are potentially harmful. It is not the dog's place to control the behavior of his or her handler, but merely to notify the handler of something of which they may not be aware. It is then up to the handler to decide whether to continue stimming, choose a different stimming behavior, or stop stimming altogether.
In some cases when a person with Autism is over-stimulated, pressure on his or her body can be very calming. Some use weighted blankets or tightly wrapped blankets to create this pressure effect. It is not always practical to carry such a blanket everywhere. However, if a person is already using a service dog anyway, the dog can be used to apply the pressure. There is some question about whether this particular function would actually qualify as a task since it doesn't really need to be trained. However, there is no reason not to use this beneficial bonus of a dog that would already be considered a service dog because of other tasks anyway.
In the last few years there has been a trend to tether very young Autistic children to dogs to keep them from bolting or running away. In some other cases the dog is expected to act as a kind of babysitter and notify the parents when the child tries to leave the house. The service dog community at large disapproves of this practice for safety reasons, however it has not yet been tested in court whether these dogs would legally be considered service dogs or not.
Video of a real autism service dog, one that is trained to perform tasks to mitigate his owner's disability, not to babysit or provide emotional support: http://www.youtube.com/watch?v=WiSB-AFnWHc