I just stumbled upon this wikipdeia article, and I really enjoyed it, so here it is again:
A banana equivalent dose is a concept occasionally used by nuclear power proponents to place in scale the dangers of radiation by comparing exposures to the radiation generated by a common banana.
Many foods are naturally radioactive, and bananas are particularly so, due to the radioactive potassium-40 they contain. The banana equivalent dose is the radiation exposure received by eating a single banana. Radiation leaks from nuclear plants are often measured in extraordinarily small units (the picocurie, a millionth of a millionth of a curie, is typical). By comparing the exposure from these events to a banana equivalent dose, a more realistic assessment of the actual risk can sometimes be obtained.
The average radiologic profile of bananas is 3520 picocuries per kg, or roughly 520 picocuries per 150g banana. The equivalent dose for 365 bananas (one per day for a year) is 3.6 millirems (36 μSv).