Having constructed and taught an Environmental Technician Training Program for a private vocational college, as well as having taught numerous HAZWOPER (Hazardous Waste Operations and Emergency Response) classes over the years, I know a little something about ionizing radiation. The main thing to remember about it is this: there is no such thing as a "safe dose" of ionizing radiation. There are instead doses which present an ACCEPTABLE RISK. Some people in government, the field of medicine, and the nuclear industry misleadingly use the term "safe dose" or the phrase "not dangerous". They should know better.
The probability is extremely high that everyone of my generation has radioactive strontium-90 in their bones. The cause of that was above-ground testing of nuclear weapons by various nations (including ours) in the 1950s. Depending upon how close one was to the explosions and numerous other factors, we "downwinders" have various amounts of the substance in our bones. What are the long-term effects? No one knows for certain. Who qualifies as a "downwinder"? Certainly everyone living at that time in both the temperate and tropical latitudes...because of global wind patterns. Those in the arctic latitudes most likely should be included as well.
We humans like to believe that in this day and age we know a lot about most everything on Earth. Well, guess what? People in the '50s felt pretty much the same way. I suspect that people in almost any modern time period felt that way. In the beginning of the Nuclear Age, it was thought that above-ground testing of nuclear weapons was "safe". I'm convinced that a small number of folks in positions of power and/or prestige knew better, but that knowledge never was conveyed to those who not long ago were characterized by the head of BP as the "small people". My point simply is this: those who now are saying that ionizing radiation from Japan will be dispersed to the point of rendering it "not dangerous" either are speaking without a sufficient level of knowledge (because no one has that amount of knowledge) or are being deceitful. I suspect it's the former, not the latter.
Various "experts", especially those in medicine, sometimes say something like: "You get more radiation from natural sources than you will from _________." They seem to think that makes __________ OK...and "safe". It puzzles me that anyone would come to such a conclusion. While it's true that we do receive some amount of ionizing radiation from natural sources, that does not mean it's "safe" to receive additional radiation from non-natural sources. No one knows the long-term effects of low doses/concentrations of ionizing radiation. No one. Such exposure may be an acceptable risk, but it's still a RISK.
A quick note on iodine tablets (usually potassium iodide). They only protect the thyroid (not other organs), and only against radioactive iodine (not cesium, plutonium, etc.).
History has shown us that we should put little stock in whatever the Government or the mega corporations say about ionizing radiation, especially if their pronouncements involve risk assessment. When their representatives claim that such-and-such ionizing radiation is "not dangerous", ALL they mean is that if you're exposed to it, you won't die vomiting and with your skin hanging in shreds. They have no way of knowing the long-term effects. Just remember one thing: "safe" or "not dangerous" only means acceptable RISK.