Hackaday wrote about an Android app by Rolf-Dieter Klein of the Helmholtz Research Center in Munich (as far as I'm able to tell):

The app works by blocking all the light coming into a phone’s camera sensor with a piece of tape or plastic. Because high energy radiation will cause artifacts on the CMOS camera sensor inside the phone, radiation will be captured as tiny specks of white light. The title picture for this post was taken from a camera phone at the Helmholtz Research Center in Munich being bathed in 10 Sieverts per hour of Gamma radiation from the decay of Cesium-137.

You can get the app here: https://market.android.com/details?id=com.rdklein.radioactivity&feature=search_result#?t=W251bGwsMSwyLDEsImNvbS5yZGtsZWluLnJhZGlvYWN0aXZpdHkiXQ..

(via Dave!)