My son had selective mutism until he was in second grade. We never considered it much of a problem until he started school, because he had completely normal speech development at home. However, he would not say a word to anyone outside of the immediate family, even people he had known since he was a baby. My pediatrician told me he was "just a little shy" and would grow out of it in time. Other than the selective mutism he was a perfectly normal little boy, he made friends easily, even without talking to them, and had no behavioral or other emotional problems and no trauma that would have caused this.
Once he got to first grade it started becoming a real problem because the teacher couldn't evaluate his progress since he wouldn't participate in class. I had consulted psychologists before, but I finally heard about selective mutism on 20/20 and it was like a light went off. I was just so relieved to know there was a name for this and hopefully help out there. We finally found a specialist in our area who prescribed medication for DS. I was really conflicted about medicating him, but felt like I needed to try anything to help him. He was on Paxil for a matter of weeks before he finally began speaking outside of the family! Once he overcame that boundary it was only a few months later and he was talking up a storm just like any other kid. He came off the medication less than 6 months later and we have had no problems since. It was like he just needed that little push to get him over the anxiety, and once he overcame the fear of talking out loud he was fine.
He just started 7th grade, and his teachers in school now have no idea that there ever was a problem. In fact, I had a small laugh to myself last year at parent conferences when the teacher told me she sometimes had to correct him for talking too much
DS is now an outgoing, social kid with lots of friends and absolutely no sign of there ever having been a problem. So there is hope for kids out there suffering from this awful disorder.