That is a radball bike(rad is german for cycle, so in english cycleball) Bike polo and radball are two different sports. Bike polo is more like hockey on bikes. It was developed in the paciffic northwest in the early 90s. Its 3 on 3 and pretty rough and faced paced. The players ride around witha mallets and hit a small street hockey ball:
http://www.youtube.com/watch?v=KvHZl8ArmNQ

Radball is much more like soccer and is played with a large ball and has to be hit with the wheels to score. The games are 2 on 2, and have no contact.
http://www.youtube.com/watch?v=iUCcuYprZj0&feature=fvst