Right now, people are buying up packets of potassium iodine tablets in a panic, bidding the price up from $10 per pack to around $540. The worst part of all of this is that many of these people are in places far away from Japan, including places such as Russia, British Columbia, and California.
There is nothing more counterproductive in a situation like this than blind panic, particularly when it's carried out by those at no risk whatsoever. (I'm looking at you, California.) In particular, the real danger here in these kinds of panic buys is in depriving those who may actually need them the most.
The CDC has correctly emphasized that people in the United States should not be taking potassium iodine in response to the events at Fukushima. Of course, this kind of hysteria is not an unknown - one need only go back to the mass panic purchases of Cipro shortly after the highly publicized anthrax letters in 2001 to find a recent example.
Nuclear Fissionary points to a hoax fallout map which has been making its rounds around the internet, stoking these very fears. Note that the radiation levels at the nearest measuring point of Ibaraki prefecture (100 miles from Fukushima) are merely elevated (i.e., detectable above natural background) - as in, there is no serious danger (levels are around 0.01-0.02 millirad per hour - in other words, quite low.) At this point, it is unlikely that any significant changes from background levels can even be measured outside of Japan.
President Obama emphasized in his speech yesterday, "The Center for Disease Control does not recommend that people in the U.S. take any precautions other than being fully informed." This is the correct response. However, one issue which has muddled the overall measured tone of the administration's response has been the directive for U.S. citizens within 50 miles (80 km) of Fukushima to evacuate - for comparison, the official evacuation zone imposed by the Japanese has been 12 km, with residents within 20 km advised to stay indoors with their windows closed. Rod Adams at Atomic Insights has been sharply critical of this move, and I think some of the rationale bears repeating.
Right now, there is an intense humanitarian crisis going on, brought about by a record earthquake and devastating tsunami. For these people - estimates put the number displaced around 380,000, finding food, clean water, and shelter from the elements where the temperatures have reached below freezing is the highest priority. Any evacuation should be weighed against the risks of displacing yet more people in a disaster area, much less the exposures created by forcing them out of their homes (where reasonable measures can be taken to protect against elevated radiation exposure, such as sealing doors and windows).
While it is my personal opinion that an evacuation of those closest to the plant was likely reasonable balanced against the risks (despite the fact that the radiation levels pose no immediate threats to human health at the plant boundary), advocating for an increase in the evacuation zone to four times the level that the Japanese government has set out (all while in the middle of a record natural disaster) seems downright irresponsible. The response of the administration and the NRC is that this radius would be standard procedure in the United States for a similar situation; however, it is extremely questionable as to the additional benefit of evacuating such a large additional number of people, particularly weighed against the additional exposure and other risks that such a larger evacuation entails.
For those curious, ANS has a special interactive page where one can see what their annual dose is from a variety of natural and synthetic sources (particularly from common medical procedures). This can help put the radiation levels being discussed in context.
NISA has also posted an English version of the latest radiation readings outside of the 20 km zone around Fukushima. For reference, 1 microsievert = 0.1 millirem (or, conversely, 1 millirem = 10 microsievert). The document also helpfully notes that the current measured average in Tokyo right now is about 0.05 microsievert/hour = 438 microsievert/year, or about 44 millirem/year. (Average exposure from everyday sources for individuals typically ranges in 400-600 millirem/year).
Meanwhile, the best advice for situations such as this was offered by Douglas Adams in "The Hitchhiker's Guide to the Galaxy": Don't Panic.