Speech language can be a successful intervention with children with special-needs including down-syndrome, and autism. Here are some relevant references and resources:

Apraxia of Speech
Square PA, (1994) Treatment Approaches For Developmental Apraxia Of Speech. Clinical Communications Disorders, 4(3):151-61.

Sharon Gretz: Using Sign Language With Children Who Have Apraxia of Speech: Availabe online at http://www.apraxia-kids.org/topics/sign.html.

Autism
Stephen M. Edelson, Ph.D., from the Center for the Study of Autism, Salem, Oregon writes:

"Many aberrant behaviors associated with autism and other developmental disabilities, such as aggression, tantrumming, self-injury, anxiety, and depression, are often attributed to an inability to communicate to others. Signed Speech may, at the very least, allow the person to communicate using signs and may stimulate verbal language skills. When teaching a person to use sign language, another possible benefit may be the facilitation of their attentiveness to social gestures of others as well as of themselves."

Dr. Edelson's article is available online at http://www.autism.org/sign.html.

Down Syndrome
Watson, Claire S-LP, (1996) Total Communication Options for Children with Down Syndrome in the Context of Hanen Programs for Parents. Wig Wag Winter 1996. Available online at http://www.altonweb.com/cs/downsyndrome/watson.html

Hopmann, Marita R., (1993) The Use of Signs by Children with Down Syndrome. Down Syndrome Today Volume 2, Number 2, 22-3. Available online at http://www.csdsa.org/artsigns.htm

Donovan, Claire S-LP, (1998) Teaching Sign Language, Disability Solutions, Volume 2, Issue 5, January/February 1998.

Miller J F, Sedey A, Miolo G, Rosin M, Murray-Branch J (1992) Vocabulary acquisition in young children with Down syndrome: Speech and sign Paper presented at the 9th World Congress of the International Association for the Scientific Study of Mental Deficiency. Queensland Australia August 1992.

Gibbs, E.D., Springer, A.S., Cooley, S.C. & Aloisio, S. (November, 1991). Early use of total communication: Patterns across eleven children with Down Syndrome. Paper presented at the meeting of the International Early Childhood Conference on Children with Special Needs, St. Louis, MO.

Reading Disabilities
Blackburn, D., Vonvillian, J., and Ashby, R. (January 1984). Manual Communication as an Alternative Mode of Language Instruction for Children with Severe Reading Disabilities. Language, Speech and Hearing Services in Schools, 15, 22-31.

Carney, J., Cioffi, G., and Raymond, W. (Spring 1985). Using Sign Language For Teaching Sight Words. Teaching Exceptional Children. 214-217.

Vernon, M., Coley, J., Hafer, J., and Dubois, J. (April 1980). Using Sign Language to Remediate Severe Reading Problems. Journal of Learning Disabilities, 13, 215-218.

Sensenig, L., Topf, B., and Mazeika, E. (June 1989). Sign Language Facilitation of Reading with Students Classified as Trainable Mentally Handicapped. Education and Training of the Mentally Retarded, 121-125.

Children in Hospital Settings
Hall, SS Weatherly KS, (1989) Using sign language with tracheotomized infants and children, Pediatric Nurse, Jul-Aug: 15(4): 362-7. Available online at: http://www.ncbi.nlm.nih.gov.