LiveTag (via slashdot)
new smart tags that harness reflected Wi-Fi signals … copper foil printed onto lightweight paper-like material … had to experiment with many different combinations of shapes, sizes, and materials for the LiveTags and run all those combinations through computer simulations. … the smart tags only work within a three-foot range of Wi-Fi devices
This is a smart hack: the foil circuit interacts with wifi signals that is around us, the interaction is detectable, and sensing acts like circuit bending to modify the wifi-signal interaction. There is a research PDF
What other radio could be affected the same way? How about near-field? Could you do this with ultrasonics?