An anonymous reader contributes a link to a BBC News article on a cheap cell-phone detector
created by six New Zealand high-school students for a business competition, excerpting "The detector, which they have called CellTrac-r, works by picking up the bursts of radio frequency activity that emit from a mobile each time it sends or receives a call or a text message. The device can detect these bursts of electro-magnetic energy up to a radius of 30 metres. It can also measure the amount of the energy to determine the distance of the mobile."
, and noting "Seems like a perfect /.er hack project, and as initiator I get 5% of gross profits."