I've built a minisumo-robot for a competition this saturday(http://www.robotchampion.se). It works quite ok, but I feel that my opponent detection is somewhat unoptimized.(Don't worry, I'll make a post with videos, pictures and such when the competition's over, no time for submissions right now, mah code needs optimizing!)
Right now I have two Sharp IR-sensors looking forward giving me a viewfield of about 120 degrees. I have an if-statement that turns left if the right sensor is closest and vice versa.