Given the fact that Japan is thousands of miles from the United States, it is highly unlikely that Americans would be exposed to radioactive material from direct inhalation of a plume from the Fukushima nuclear complex.
While wind patterns will likely carry the radioactive plume eastward, radioactive material will be so diffuse by the time it reaches Hawaii, Alaska, or the mainland United States that it is highly unlikely to create significant health concerns.
Related to this, UCS just released a statement about potassium-iodide pills:
The people of Japan should be given priority access to potassium iodide (KI) pills used to protect against thyroid cancer following inhalation of radioactive iodine.
Given the fact that Japan is thousands of miles from the United States, it is highly unlikely that Americans would be exposed to radioactive iodine from direct inhalation of a plume from the Fukushima nuclear complex. Direct inhalation is the kind of exposure that potassium iodide pills would be most effective against.
Regardless, there are reports that global supplies of potassium iodide pills are being depleted because Americans are buying them, prompting fears that there will not be adequate supplies in Japan in the event of a larger radiological release.
Besides inhalation, another way Americans could be exposed to radioactive iodine is if agricultural products were contaminated. Radioactive iodine could be ingested by dairy cows, for example, and then would be concentrated in milk. Potassium iodide, however, would not be an effective countermeasure in that situation. Moreover, federal and state health authorities would test for such contamination and could take products off the market if necessary.