The nuclear fallout from a possible Japanese Nuclear facility explosion has many people scared. Here in Texas, people are buying up all the potassium iodide because of possible meltdowns in Japan. Japan is over 5000 miles away from the coast of California. Any radiation that gets into the atmosphere will be dissipated to very low levels by the time it reaches the U.S. Some will get here just because the prevailing winds will push things around in the upper atmosphere but I don't see any worry yet.
Potasium Iodide is used when thyroid radioiodine particles are expected. The iodide floods the thyroid to prevent radiation from penetrating and causing cancer and sickness. Its very effective but not meant to be taken on a regular basis. Prolonged use can lead to organ damage (MSDS). Its meant as a short term survival tool and could cause more harm than good for people to take it if they think radiation poisoning is an issue from Japan. Potassium Iodide will not protect against other cancer causing radioactive particles. The best know method to prevent from getting radiation poisoning of any kind is time, distance, and shielding. The more distance and shielding the less exposure you receive. Time dissipates the radioactive molecules as they decay.
MSDS-Potassium Iodide. Sciencelab.com