Not to be rude, but I work with kids with autism and I have to say that this question is kind of annoying to us in the biz. The incidence of savantism is very rare. Rain Man was based on a fellow named Kim Peek who had a disorder which wasn't identified as autism.
Generally, individuals with ASD are very focused on order and categorization of objects and events, so they often have strengths relating to this as mentioned by walleywonka. The date and time thing is kind of neat and is very uncommon.
I don't want to bust your chops for asking the "Rain Man" question in this AMA, but if you meet a parent of a child with ASD somewhere, please don't ask them if their child has some special super ability. Ask them what their kid is in to, what makes him or her happy, things you would ask about a typically developing child. ASD kids are like every other kid, with strengths and weaknesses, but they're often more pronounced than you see in a typical person. The Rain Man thing will pretty much 99% of the time get you a "no" answer and even if it's a "yes" answer, the kid generally has so many other struggles and challenges that the people who live with him or her don't spend too much time bragging up the parlor tricks associated with incredible memory or attention to detail.