I read about this and I think the survey had at least minimal integrity, even though it was a phone survey. If I recall correctly, they asked a bunch of general health questions with a few about autism spectrum disorders thrown in the mix. The figure sounds fairly believable to me, not necessarily because more people are autistic (although that is a possibility), but because we know so much more about it now. My son was recently diagnosed with AS, and as we learned about it, came to realize that my husband is very likely an Aspie, and I share many traits as well (may even be an Aspie too, who knows). Now there are suddenly 3 (possibly) people in my house with AS that didn't technically have AS last year. It's not hard to understand how a spike could occur.
One sidenote of the study that I found very interesting was that 40% of parents who stated that they had a child who had autism claimed that the child was no longer autistic. I don't know if these parents were included in the original statistics or not. I don't think that they were, but either way, really?! On second thought, maybe the study was total garbage.