I've heard this before and have done a little reading on it. The symptoms of the two can be very similar, but they are very different.
http://www.steadyhealth.com/articles/Schizophrenia_and_Autism_a330.html
The problem with this article and with the DSM in general is that it's vague. Nobody knows what autism, or Schizophrenia is, so they are categorized based solely on behavioral traits. It's pretty well known that Schizophrenia is thought to involve excess Dopamine circulation, but that is not involved in a diagnosis.
To put it another way, the symptoms of the two disorders don't have to be very different. The same person could be diagnosed with either a PDD or Schizophrenia depending on which doctor he sees and which symptoms he emphasizes. The main criteria someone would use to distinguish between those two categories is the presence of positive symptoms like hallucinations. The problem is there are some types of Schizophrenia that don't include positive symptoms.
Seems like half (or more) of the frickin' disorders can masquerade as PDDs, not just the psychotic disorders.
If you're depressed, you'll have reduced affect. If you're anxious, you won't maintain normal eye contact. If you have trauma or a long-term mental illness (or both) you stand a good chance of having been undersocialized and/or having lost a lot of friends and not have much of a social group and/or having learned to hate people at some point and thus be somewhat isolationist and have social skills that are atrophying. (Also, your family members will remember you having had 'problems' since childhood.) If you have a hangover you have sensory issues. Same with hallucinogens. If you're on downers your affect is fucked up. If you're on virtually *any* substance your social skills are altered, your affect is fucked up. If you're trying to distract your therapist you might go on forever about your interests to avoid talking about anything too close to home.
There are about a billion other iterations, with more symptoms, substances, circumstances and disorders. To a hammer everything looks like a nail. Short of neuropsych testing, it takes more than a quick run down a symptom checlist to get an accurate idea of what's happening. Supposedly the DSM doesn't imply etiology (except with RAD), but given current knowledge of PDDs, you don't expect the same interventions to do the same things for someone with an ASD versus someone who is undersocialized and depressed. Yet, they could have the same exact damn symptoms, at first (and might self-reort a comperable history).
...I totally did not just go off on a rant for incredably obvious reasons (to folks who know me, anyway). 
An anxiety disorder could produce symptoms like bad eye contact, bad social skills, and even flat affect. But the clue to something more pervasive than an anxiety disorder (and this is my theory, not based on anything I can remember reading) is
bad coordination. I don't just mean physical coordination, but physical and cognitive coordination, and probably other types of coordination. It's sort of like having the potential to be verbally lucid or physically agile, but different processes don't connect right, and something that should be simple ends up being very laborous. Sometimes other people don't notice, especially depending on the age of a person. For example, when I was a kid, I had an awkward gait when runnning. Now I can run normally, but it takes a certain amount of effort to move my limbs the right way. I know it isn't normal, because if I didn't use that effort, I would run unnaturally.
