I have been doing some more research into high school and middle school CS / programming competitions. As Anthony found, the two really big ones are USACO and ACSL. I've been talking to a lot of people here at CMU and I've found that some of them have done USACO and fewer have participated in ACSL. One interesting thing that I've found is there are many, many local competitions hosted by local universities or organized by some other party. If ACSL or USACO was not offered at a student's high school, many were involved in local competitions.
A lot of the students I know who have been interested in programming from a young age participated in online contests like Google Code Jam or other programming competitions. Unfortunately, these competitions have a heavy focus on only programming rather than algorithms and other parts of Computer Science.
Other competitions that are not just CS competitions are Robotics competitions which are all over the United States. FIRST has Robotics competitions for students from elementary school through high school. While it is not strictly CS (a large portion of many of the contests in engineering, most do require problems solving and programming). One of our hypotheses was that if CS is associated with math instead of engineering, it will be more attractive to women. I would be interested in looking at whether participation in Robotics competitions which contain a major engineering component as well as a CS component had any effect on women's decisions to major in CS.
I would definitely like to put some questions related to these competitions on some of our surveys. Find out how many people participated in CS / programming competitions and of what kind. ACSL / USACO, local competitions, online, Robotics, or some other kind?
I would also be interested in examining whether students participated in math competitions. Almost everyone I know in CS participated in some sort of math competition or math olympiad in high school. I would also like to look at it from a high school perspective. What percentage of students who participate in high school math competitions go on to major in CS? What percentage of students who participate in high school CS / programming competitions go on the major in CS? Robotics competitions?