A couple of California newspapers (other Google News links are broken) have short articles about a research report out of UC Davis saying that autism diagnoses in California cluster in areas where the families are older, whiter, and better educated.
I couldn't find the paper itself, but this blog has an interesting discussion of it, including noting earlier similar research.
If it were among poor people, we would assume there was something in the environment causing it, as lead consumption is higher among poor kids than rich kids. But this looks like a bias in diagnosis: kids of older, whiter, better educated parents are more likely to get an autism diagnosis than kids of younger, browner, less educated parents. This means not that autism clusters in rich areas but that it more often goes undiagnosed in poor ones.