The New York Times report that a radiation plume will reach California by Friday has a lot of people asking us for more information about what to do. (See http://www.nytimes.com/2011/03/17/science/17plume.html?_r=1) Fortunately, the amount of radiation that would actually fall on California (or Oregon, Washington, British Columbia, Alaska, etc.) is currently very small. Too small to pose a serious health risk for right now.
The time to get really concerned is if -- and only if -- the Fukushima reactors suffer a fuel rod "meltdown" event which would release a massive amount of radiation into the atmosphere. The United Nations has calculated that such an event would send a cloud of very powerful radiation headed eastward, arriving on the shores of the United States and Canada within 2-3 days after the meltdown, depending on wind patterns.
Importantly, this will give you some time to take action should such an event occur. The next 24 - 48 hours are crucial in Japan's effort to stop the nuclear fuel rod meltdown. It is entirely unknown whether Japan can succeed in stopping it. The situation remains dire.