A reader pointed out the possible connection between autism and Vitamin D, and so I followed up on it. Here is a nicely-written article from the Vitamin D Council that sketches out the theory. They point out that currently this is only a theory, and that more research is needed.
However, it does provide a global concept for what causes autism. This idea was recently written up in a journal and the abstract is here. It will be interesting to see how this line of research develops.