From Dave Griscom:
Cesium-137, a gamma ray emitter, is the worst. Fortunately, what arrives in the U.S. is likely to be highly diluted and may not be a great health problem …unless a large amount of dust containing Ce-137 is aspirated or gets into the food chain (it’s chemistry is the same as that of potassium).
However, if the government continues to block web access to prevailing ambient dose rates near where you live, you should seek alternative means of finding out (like buying a Geiger Counter).
A nice primer on radiation and radioactivity can be found here:
Radiation and Radioactivity
Meltdowns Grow More Likely at the Fukushima Reactors
March 13, 2011 · By Robert Alvarez