Amanda Baxter (Queensland Centre for Mental Health Research in Australia) and her colleagues have put together a study entitled “The epidemiology and global burden of autism spectrum disorders”. You can read it here (warning PDF).
In 2010 there were an estimated 52 million cases of ASDs, equating to a prevalence of 7.6 per 1000 or one in 132 persons. After accounting for methodological variations, there was no clear evidence of a change in prevalence for autistic disorder or other ASDs between 1990 and 2010. Worldwide, there was little regional variation in the prevalence of ASDs. Globally, autistic disorders accounted for more than 58 DALYs per 100000 population and other ASDs accounted for 53 DALYs per 100000.
In other words, autism only appears to be increasing because of better awareness.