Most credible agencies insist it's not only a waste of time, but in fact could be more danger than it's worth. Including:
The California Department of Health
("Radiation from Japan is expected to be thousands of times less than daily background radiation from natural and man-made sources—like the sun, air, soil, medical imaging, and life-saving therapies.")
The Johns Hopkins Office of Critical Event Preparedness
(""CEPAR experts note that harmful levels of radiation are not expected in the United States as a result of the nuclear crisis in Japan, and thus, taking the medication as a preventive measure is wholly unnecessary.")
The Washington State Department of Health
("We do not advise the public to purchase KI. It’s not an “anti-radiation pill;” it’s only helpful in very specific conditions, and protects only against radioactive iodine."
- also this big chunk -
"KI protects the thyroid against high concentrations of radioactive iodine, which is a type of radioactive material that is very unlikely to make it to the upper atmosphere. If it were to get in to the upper atmosphere, by the time the winds blew it from Japan to other parts of the world such as Washington, it would be at such low levels that it would pose no health threat to people. Levels would be diluted by wind and distance. Potassium iodide is typically given to people who are very near the source of high levels of radioactive iodine, such as nuclear plant workers or residents near the plant who may not be able to get out of the area soon enough after a nuclear incident. In Japan, for example, the evacuation zone is within 20 kilometers, or about 12 miles, of the plant. We’re thousands of miles away. The state Department of Health recommends against KI for people in this state concerned about radiation from the Japanese nuclear event.")
Actually this FAQ is pretty good.
In short, no.