I read the following in USA Today:
A decade ago, one in every 2,500 U.S. children had autism; now it’s one in 166, the Centers for Disease Control and Prevention says.
But is it true? Autism Diva (a new blog) raises some serious doubts. Autism may still be rising, but this number may be the latest "urban legend." Using Google alone, I could not track down the ultimate source (beyond the CDC press release) of the "1 in 166" claim. I can, however, tell you this. A search for "autism 166" yields (repetitive) riches on google.com, but nothing real on scholar.google.com.
Addendum: Here is one serious piece; it supports my skepticism. Thanks to Daniel Starr for the pointer.