There is a lot of conflicting information about radiation releases from power plants in Japan. It's a keen health concern for millions of people around the world, including me, and my hope with this post is to add a little clarity where I can. Here are some key points.
Today's maximum reported radiation release of 400 millisevierts is equivalent to 40 rem in the US system of measuring these things. That's about the maximum you could tolerate in a month. A few people would experience nausea, vomiting, and, about a month later, lower white blood cell counts.
Potassium iodide only protects against radioactive iodine, not other radioactive heavy elements like cesium and strontium. And it only protects against thyroid cancer. Protecting against thyroid cancer is a good thing, but you should neither suppose that having KI on hand is all you need or that you necessarily need to rush out and get it.
It's easy to forget that there have been hundreds of much larger releases of radiation into the atmosphere during the era of above-ground testing of nuclear weapons. Humankind survived. I don't mean to minimize any of the risks, but it's not time to panic yet, outside of the nuclear disaster zone in Japan. Comments are welcome.