I used this design in a PIC Microcontroller class I taught to other engineers at work.  This picture on the right is a view of my robot that eventually went head to head with the other one, below.  I also took this one down to the San Francisco Robotics Sumo competition.  It had a tough time because of it's high center of gravity, but I was just happy to win one of the matches.  

Using an IR distance sensor, p/n GP2D02, it is able to scan the area and look for other robots.  I got these sensors from Acroname.  The front "scoops" are for lifting the other robots' wheels off the ground.

This is another student team's robot. The scoop is a modified floppy disk jewel case.  These robots use a servo for each drive wheel, and two power sources: a 4xAA pack for motor power, and a 9 volt for microprocessor power.  Using Micro Engineering Labs programmer, changes can be downloaded with the chip in place, making it an easy "load & go" system.