[greencactus3]

the IR trans/recieve of the lego mindstorms rcx brick is quite easy to use..
and another idea would use servos and rc controllers. cimply put, both robots hold a controller. they manipulate it and the rc reciever turns those signals into motion of servos. then sensors read the movement of the servos. (Variable angles.... so pots? maybe?) and well the more channels the radio equipment, the more info each robot can send and recieve at once... so well.. kinda like a 2 way radio... using readily available items.. ($10 shipped for a simple 2 channel AM radio transmitter+reciever+crystals off ebay.)