I believe the Autistic person is capable of an unimaginable level of intelligence. I will probably not be able to properly convey what I see, but I will try.
They have an awareness and focus that is very very great. Imagine being in a crowded bar with your friends. There's loud music, dancing, pool, laughing, loud obnoxious behaviours, etc going on all around you. For most people, though it is loud, the background can become white noise as you focus on hanging with your friends. Now try to imagine being able to apply that same level of focus to everything going on around you and be consciously aware of all of it, at the same time. If you can somehow actually picture that, now try applying the same focus simultaneously to all happenings in crowded sporting arena. What we do to focus, is their average awareness. Now if that is simply their awareness, try to imagine how intense their focus can become.
[In regards to the more severely Autistic...] Their minds operate at a much more advanced level than the average human can even begin to comprehend. Because they're initial existence is primarily cerebral, motor control may become difficult as a result. Their minds had been so busy contemplating other things, they had not yet had the time to learn how to perform some of the most basic bodily functions, let alone fine tune their motor control. In it's current form, it is a developmental disability. Different functions of the brain develop at different rates depending on where the focus and the peripheral awareness is applied.
In algebra, if you have an equation with both x and y variables in it, it becomes difficult to solve for y, without knowing what x is. An autistic will get stuck on trying to figure it out. Because of their intense focus, this can take a while. This may seem like they're "slow", but what we fail to realize is how much processing the Autistic brain has computed to solve for y. It could take decades for the average brain to consider so many possibilities. It's like comparing a Cray supercomputer to a single Pentium I powered desktop PC.
They spend a lot of their lives trying to figure the world out for themselves. They are also divergent thinkers, experiential learners. Traditional teaching methods and sequential verbal language are either too slow and/or inadequate to provide the answers for Autistics to grasp what they are wanting to understand. This is the key really, if we knew what they wanted to understand, and we found a way to help them learn it in a way that makes sense to them, they would probably not only develop in every way faster than they do now, but probably far faster than the average human could keep up with.
The challenge in getting through to them is because of their intense focus. Think of it this way, you are watching the superbowl (or stanley cup, world cup, etc event) and your wife wants you take out the trash or clean the garage or whatever, but you ignore the request because you are watching the superbowl. You still heard your wife ask, but that only registered in your peripheral awareness, because you are focussed on the game. You might do it later, after the game's over, or you might have forgotten because of how long the game went on for. Causing your wife to get mad because you didn't do what she asked you to do. For an Autistic person, this superbowl can go on for a very long time.
Some insightful links:
http://www.youtube.com/watch?v=U_GXVzZ0Unk
http://carlysvoice.com/home/faq/
). I posit that most learning disabilities, developmental disorders, personality disorders, mood disorders, etc are all related to the same "thing". I can almost prove it, but I have to spend 10-20 years going through University to prove it in a socially acceptable way. :S (And I don't mean through crazy drug experiments, or cutting people open or anything like that. My theories are based on very non-invasive methods. 
