Well, the nuclear crisis in Japan seems to be causing a run on potassium iodide (KI), and not just in Japan. If news reports are to be believed, people in many other regions (such as the west coast of the US and Canada) are stocking up, and some of these people may have already started dosing themselves.
Don't do that. Don't do it, for several reasons. First, as the chemists and biologists in this site's readership can tell you, it's not like KI is some sort of broad-spectrum anti-radiation pill. It can protect people against the effects of radioactive iodine-131, which is a major fission product from uranium. It does that by basically swamping out the radioactive iodine a person might have been exposed to, keeping it from being taken up into the body. Iodine tends to localize in the thyroid gland, and that uptake and local concentration is the real problem. An unfolded newspaper will shield you just fine from the alpha particles that I-131 gives off, but not if it's giving them off from inside your thyroid. Correction: I-131 is a beta/gamma emitter - my apologies! The point about not wanting it in your thyroid, of course, stands. . .
And this is why potassium iodide won't do a thing to help with the other radioactive isotopes found in nuclear reactors. That includes both the uranium and/or plutonium fuel, as well as the fission products like strontium-90 and radioactive cesium. Strontium-90 is a real problem, since it tends to concentrate in the bones (and teeth), and it has a much longer half-life than I-131. Unfortunately, calcium is so ubiquitous in the body that it's not feasible to do that uptake-blocking trick the way you can with iodide. The only effective way to deal with strontium-90 is to not get exposed to it.
Another good reason not to take KI pills is that unless you're actually being exposed to radioactive iodine, it's not going to do any good at all, and can actually do you harm. Pregnant women and people with thyroid problems, especially, should not go around gulping potassium iodide. Nothing radioactive is reaching North America yet - there's the Pacific Ocean to dilute things out along the way - which makes it very likely that more people on this side are in the process of injuring themselves by taking large unnecessary doses of iodide. This is like watching people swerve their cars off the road into the trees because they've heard that there's an accident fifty miles ahead.
Now, if I were in Japan and downwind of the Fukushima reactors, I would indeed be taking potassium iodide pills, and doing so while getting the hell out of the area. (That last part, when feasible, is the absolute best protection against radioactive exposure). But here in North America, we're already the hell out of the area. The only time to take KI pills is when a plume of radioactive iodine is on the way, and that's not the case over here. We'll have plenty of notice if anything like that happens, believe me - any event that dumps enough radioactivity to make it to California will be very noticeable indeed. Let's hope we don't see anything of the kind - and in the meantime, spare a thought for those reactor technicians who are trying to keep such things from happening. Those people, I hope, will eventually have statues raised to them.