The attachment was used to pick up rings in one mission. Last Year we had a student that came up with this design that works really well. He installed a lego rubber band to a lever and with a push you can slide it on and off. The one shown did not need the rubber band lever.
With this method you can chage attachments quicly. We also put the missions in my blocks and add a wait until button pushed between them so the missions change automaticaly with out searching through the list on the NXT.
View our Smart Moves robot in action http://www.youtube.com/watch?v=x3FCQtOBqRI