Earlier this month, we reported on the Scosche RDTX-Pro that connects via a dock connector to turn an iPhone or iPod touch into a radiation detector. That device is set to go on sale in Japan from next month but if you're not in Japan or just don't want to shell out extra cash on any peripheral hardware, then the WikiSensor app might be worth a look - it won't be as accurate, but the only extra bit of kit you'll need is some opaque black tape.