I know that this discussion is focused on the VEX sonar but, the Matbotix sonar sensor is a much better sensor. It has been redisigned and is offered in several versions with different cones of detection. The nice thing about programming these sonars is that they offer a analog voltage output. A simple A to D call is all that is needed. They are 25$ plus shipping. Check out Sparkfun electronic for the differnt models and some links to FAQ and data sheet. The Sharp IR proximity sensor are similiar to the sonar but, better for some uses.
http://www.sparkfun.com/commerce/cat...hp?cPath=23_84