I've added two of the Pololu IR Proximity sensors http://www.pololu.com/catalog/product/2460
one on each side of one of my Zumo robots.
These are connected to pin 6 on the right, and A1 on the left. That allows me to detect when the robot rolls past another robot in the ring. It was really easy to add on, and the coding was pretty simple. I now need to fine tune the algorithm now to decide what to do. Currently I reverse the robot a small amount, and then turn in the direction of the detected robot. It's difficult to see yet if it give this one an advantage over the other one which is simply following the standard Border Detect routine.