I just finished installing some hardware for SparkFun's robot tracking system. It's sort of a mixed reality system combining home made robots and hacked together control systems, IR tracking and sweet high res projections from above.
It uses a modded PS3 eye, an ultra-short throw projector mounted above the ceiling, Community Core Vision and Processing running MSAFluid.
The robots can consist of pretty much anything, all that the robots need to be tracked is a simple IR LED pointing upward towards the modded PS3 webcam which now only sees infrared. In this case Tim has attached a small throwy style cell battery, resistor, switch and IR LED together and literally just taped it to his robot.
Tim's sumo robot recently competed and placed at Robothon in Seattle, but now is controlled via a hacked Wiichuck connected to an Arduino and xbee for wireless control.
The InfraRed tracking is done by the PS3 eye with the IR blocking filter removed and an IR filter that ONLY allows infrared light. The result is an all black image with only the IR LED showing up in the webcam. The webcam is directly above projected image hidden in the ceiling.
Community Core Vision is a great piece of free software with a calibration system which has mainly been used for multi-touch screen, but it can also be used for other types of tracking. CCV can output the tracked points via Open Sound Control to Processing...just like TouchOSC in my last project. Or it can output to Flash, which is also really cool, but I don't know much Flash.
Processing can read the OSC signals and interpret them in any number of ways. My favorite easy to tweak demo is Memo's MegaSuperAwesomeFluid TUIO particle system. I just barely modded Memo's code to fit the size of the projection and the speed of the particles emitted from the front of the robot.
That's about it for now, there is definitely more to come. Also if anyone wants to come check out this project or just hack some electronics with us...SparkFun is having a free electronics hacking meetup tomorrow, Dec 5th from 9:30am-2pm in Boulder, CO. Hope to see you there!