It seems like everywhere I look, there is more research to support a link between environmental factors and autism. Over the summer, a twin study was published, in which it was discovered that twin siblings have a higher rate of autism than their non-twin siblings. This indicated that environmental factors play a role in autism. Here's a link to a New York Times article which explains the results of the study in easier to understand terms.
I just read about a Norwegian study that found that taking folic acid supplements prior to pregnancy decreases the chance of the child having a severe language delay at age three. As we all know, language delay is one of the main indicators of autism.
Recently, an American study found that taking prenatal vitamins 3 months before and one month after pregnancy led to a decreased risk of autism.
As with all research studies, caution should be taken when interpreting the results, but nonetheless, these studies linking possible environmental factors are interesting, and I look forward to future studies about other factors that may play a role in autism.