This will be a short diary. I have seen several comments in various threads today from people in Hawaii and the West coast wondering if they are in danger. The short answer is no. People in Japan are in danger, and with a further deterioration of the situation at the failing reactors, it could become catastrophic. This diary isn't about that. This is strictly about the larger risks beyond Japan.
The worst nuclear disaster ever was Chernobyl, and it has been well studied. You can read up on Chernobyl at Wikipedia. There is also another site, here, that has a map of the radiation plume that extended from Chernobyl.
But here is the short version. No significant radiation from Chernobyl went further than 800 km from the site of the reactor. Bad as Fukushima may get, it will likely still be less bad than Chernobyl (though still really, really fucking bad for people in Japan).
So, the maximum extent of serious danger is 800 km (downwind).
Hawaii is about 6000 km from Fukushima.
The West Coast is about 7,000 to 8,000 km from Fukushima.
The radioactive plume will simply be far too dispersed to cause any concern for people living in Hawaii or the west coast of North America. So don't start taking huge doses of Iodine or Calcium, and don't lock your kids in their rooms under wet towels.
There is a potential catastrophe brewing in Japan, but it is in Japan. The best thing we can all do is send our money and support to the Japanese--cause things may be getting very bad there before this is all over.