I'd say the most important aspects of creating a character with autism is to start with "differential diagnosis" - know what autism isn't - since it's such a varied and complex disorder. Autism is a part of the spectrum of diagnoses known as pervasive developmental disorder (many publications like Time and Wired will get this wrong, incorrectly stating that PDD is a form of autism). Also, high-functioning autism is distinguished from Asperger's Syndrome (Know any really nerdy, socially inept CPAs or engineers? There's a good glimpse of Asperger's.) Most importantly, high-functioning autism is not not NOT Dustin Hoffman from "Rain Man." That is a stereotype derived from a form of PDD known as "Savant Syndrome."
Individuals with high-functioning autism vary quite considerably. Some experience a good deal of difficulty with academics, while others are quite good at one or more areas. With autism, unlike Asperger's, individual's social deficits can rise quite beyond mere social ineptness or a lack of empathy. I have one teen on my case load who simply doesn't understand why it isn't acceptable for him to steal money from his parents or siblings or beat up his sister when he's angry. He doesn't get why he has to adhere to schedules, go to school, or clean himself, or why there are negative consequences for these things. However, just because someone has socially avoidant behavior doesn't mean they have autism!
Social and emotional connections simply don't particularly happen for a lot of these kids. While some might be completely detached from schedules, others might be rigidly dependent on them (not Adrian Monk OCD). Transitions, such as from a favorite activity to going home, can be hard. Asking one of these individuals why something is happening, or what they should do if something occurs, typically garners a rote or very literal response. Many have difficulty with their receptive or expressive language, or with complex themes. Some display "stimming" behaviors (rocking, head-banging, fluttering pieces of paper), but typically I've seen that more often with lower-functioning individuals.
There are some very interesting new facts being discovered these days. For example, many individuals with autism have either severe sensory deficits (hence stimming or needing deep-pressure to calm, for example), or highly sensory sensitive (such as to the flickers in halogen lightbulbs). Many have vision problems or severe allergies to things like wheat, gluten, soy, and so on. Most interesting for me, since I work primarily with teens and young adults with autism, is seeing the increasing incidence of comorbid mental health diagnoses. The majority of my high-functioning clients with autism have some sort of personality or mood disorder. Most common are depression (either bipolar with manic episodes or with psychotic features), schizoaffective disorder (sort of like a mood disorder but with more pronounced schizoid symptoms) and schizophrenia. Interestingly enough, I have one consumer who appears to have histrionic disorder (though I believe she was misdiagnosed and most likely has Asperger's, not autism).
For the single best portrayal of an individual with autism in fiction, read Mark Haddon's "The Curious Incident of the Dog in the Nighttime." Bryna Siegel's "The World of the Autistic Child" is a very good resource. You might also want to get your hands on a DSM (preferably IV-TR; that's the Diagnostic and Statistical Manual of Mental Disorders). All of these are available at your local bookstore, or should be.
[I am a social worker for California's regional center system, serving children and adults with developmental delays. Prior to that, I was a special education teacher. I currently have several individuals with high-functioning autism on my caseload.]