There's a particularly remarkable study linked to by this article (but the anchor text is misleading so the link is hard to find) to a study of the long-term health effects of Chernobyl, which was so bad that it could almost be considered a "dirty bomb". From the abstract:
Among adult populations, there is no strong evidence to suggest that risk of thyroid cancer, leukaemia, or other malignant disease has increased as a result of the Chernobyl accident.
The article also says that in a study of 120,000 Hiroshima and Nagasaki survivors, some of whom were exposed to the blasts and other who were away at the time, that since 1950 there have been 822 "excess" deaths among the exposed population, or about 2% of the 42,304 people who have died during the study period.
Yes, the situation in Japan is very serious and very tragic. But can we please stop freaking out about radiation?