Research on the Benefits of Children and Play:

Kuo, Frances E. and Andrea Faber Taylor (2004). "A Potential Natural Treatment for Attention-Deficit/Hyperactivity Disorder: Evidence From a National Study." American Journal of Public Health, Vol. 94, No. 9.
Kuo, Frances E. and Andrea Faber Taylor (2000). "Children with attention deficits concentrate better after walk in the park." Journal of Attention Disorders, 12, 402-409.
Nauert, Rick, PhD. "Leisure Play is Important for Human Collaboration." Psych Central News. 17 April 2009.
Taylor, Andrea F., Frances E. Kuo and William C. Sullivan. 2001. "Coping with ADD: The surprising connection to green play settings." Environment and Behavior, Vol. 33, No. 1 (January), pp. 54-77.
White, Randy. "Benefits for Children of Play in Nature." White Hutchinson. 8 Nov. 2008.
Whitehouse, Sandra, James W. Varni, Michael Seid, Clare Cooper-Marcus, Mary Jane Ensberg, Jenifer R. Jacobs, and Robyn S. Mehlenbeck (2001). "Evaluating a Children's Hospital Garden Environment: Utilization and Consumer Satisfaction." Journal of Environmental Psychology, Vol. 21, No. 3, pp. 301-314.
Luckily, new research on Autism and children's environments as well as the benefits of play is popping up all the time, please check back for the latest studies!

Research on Autism and Children's Environments:

Hebert, Bonnie B. (2003). "Design Guidelines of a Therapeutic Garden for Autistic Children." Master of Landscape Architecture Thesis for Louisiana State University and Agricultural and Mechanical College.
Mostafa, Magda (2008). "An Architecture for Autism: Concepts of Design Intervention for the Autistic User." International Journal of Architectural Research, Vol, 2, No. 1, pp. 189-211.

Remington, Anna, John Swettenham, Ruth Campbell, and Mike Coleman (2009). "Selective Attention and Perceptual Load in Autism Spectrum Disorder." Psychological Science.
Yuill, Nicola, Caroline Roake, Ruth Aspden, and Brenda Todd (2007). "Designing a Playground for Children with Autistic Spectrum Disorders: Effects on Playful Peer Interactions." Journal of Autism Development Disorders, Vol. 37, No. 6.