You are right!
Usually these children have IQ less than 50 which means they can not depend on them selves and can't be educated but God gave them extraordinary abilities which a genius person can't do it!
Around 10 per cent of people with autism show special or even remarkable skills. The skills range includes:
Splinter skills - the most common type. The person, like an obsessive hobbyist, commits certain things to memory, such as sports trivia.
Talented skills - the person has a more highly developed and specialised skill. For example, they may be artistic and paint beautiful pictures, or have a memory that allows them to work out difficult mathematical calculations in their head.
Prodigious skills - the rarest type. It is thought that there are only about 25 autistic savants in the world who show prodigious skills. These skills could include, for example, the ability to play an entire concerto on the piano after hearing it only once.
In all cases of savant syndrome, the skill is specific, limited and most often reliant on memory. Generally, savant skills include:
Music - the piano is the most popular instrument. For example, the skill may be the ability to play the piano without being taught.
Art - such as the ability to draw, paint or sculpt to high standards. For example, Richard Wawro is an autistic savant who is also blind, but his crayon drawings command up to $10,000 each.
Mathematics - for example, the ability to work out complicated sums in their head, or to calendar calculate (for example, work out what day it was on 1 June1732).
Language - in rare cases, the person may be unusually gifted in languages.
Other skills - such as knowing the time without seeing a clock, untaught mechanical skills, having an unfailing sense of direction or the ability to commit maps to memory.
I saw a case in real live here in Sudan, this boy had mathematics skill and he can able to calculate huge numbers in seconds and also can remeber every day in every given year 'What day of the week was May 22, 1961? and he can determine the answer within seconds: "Monday". !!
But the puzzling question here is how such a mentally retarded person can be able to have such specialized skills?!
There are a lot of theories available today explain the reason and there are alot of researchs done in this case
also they discovered that someone can acquire such skills when he involves in head trauma.
There are also a lot of movies and stories done in this case without giving it's real name and they always present the genius part of these persons!
Check the wiki's article about this disease http://en.wikipedia.org/wiki/Autistic_savant
Also read this article with the real life rain man kim peek videohttp://purplemedicalblog.blogspot.com/2006/11/autistic-savanttwo-seconds-to-discover.html
Also this is summerized article about the diseasehttp://www.answers.com/topic/savant-syndrome