One of the biggest nightmares of the police and security services is a dirty bomb, this is where a terrorist blows up a conventional bomb surrounded with radioactive material. This is not a real nuclear bomb, but could make a small area radioactive for a long time, and create a huge amount of panic. The real headache is that without special equipment is is very hard to tell if something is radioactive so you could walk down a street with a bag full of nuclear waste without being noticed.
Researchers at Purdue university and a consulting scientists called Andrew Longman have come up with a possible solution by taking advantage of mobile phones. The idea is to put a small cheap solid state radiation detector in as many mobile phones as possible. Many mobile phones especially in the US have positioning devices built into them, so if a phone detects more than a certain level of radiation it can automatically call into a base station and report the radiation level and its position.
Andrew has developed software that will put together hundreds of these reports and only give a warning if several phones detect radiation which is in the same place or moving at a sensible speed and could be from the same source.
The whole system would be dependent on getting mobile phone manufacturers to put detectors into enough phones that you can build up a useful network of detectors, not picking up legitimately radioactive things such a people undergoing a course of nuclear medicine, and persuading people they want to carry a phone that is reporting back to the government all the time.