In "Oxytocin receptor (OXTR) does not play a major role in the aetiology of autism: Genetic and molecular studies," Tansey et al. (2010) write (abstract only):
"Oxytocin (OXT) has been hypothesized to play a role in aetiology of autism based on a demonstrated involvement in the regulation of social behaviours. It is postulated that OXT reduces activation of the amygdala, inhibiting social anxiety, indicating a neural mechanism for the effects of OXT in social cognition. Genetic variation at the oxytocin receptor gene (OXTR) has been reported to be associated with autism...Data presented here does not support the role of common genetic variation in OXTR in the aetiology of autism spectrum disorders in Caucasian samples."
Interesting. Of course, there's lots of work to be done, and sample sizes have been small in all the studies, either in favor of a a genetic variation or disconfirming a link.