What has happened is that the definition of autism has been broadened as to be almost unrecognizable. This makes the number not credible, IMO.
Now a lot of behaviors are diagnosed PDD possibly in the autistic spectrum. It has broadened the diagonis a little too far I think, so those truely affected aren't taken as seriously. My nephew is autistic. I love him dearly and hope that with the therapy he is recieving he will be able to lead a somewhat normal adult life, but I know there is only a slim chance of that.