For more reliable info and status of Japan's nuclear plants, see:
1. Nuclear Energy Institute: http://www.nei.org/newsandevents
Click on the "information on the Japanese earthquake and reactors in that region"
This also gives links to other sites.
2. World Nuclear Association: http://www.world-nuclear.org
Also see their info sheets, such as 'nuclear power plants and earthquakes'
3. Japan Industrial Forum: http://www.jaif.or.jp/english
They give reports on the plant status of each plant.
Radiation measurements: 1 Sievert= 1 Sv= 100 rem, or 1 mSv = 100 mrem.
Normal background doses for a person in the US is around 500-600 mrem/year.(5-6 mSv/yr)
A dose of 40 rem (400 mSv) or less produces no detectable effects, such as increased cancer incidence.
Doses above 40 rem increase your cancer incident rate proportional to the dose. A dose of 100 rem increases your cancer incident rate by about 5%, and a dose of 200 rem increases the cancer incident rate by about 10%.
A dose of about 100 rem (1000 mSv or 1 Sv) would induce radiation sickness. Most people would recover from such a dose with treatment.
A dose of about 400-500 rem (4000-5000 mSv) would be lethal to about 50% of the people.
The Japanese plants do have a problem, but it is pretty localized to the plant site and to a lesser extent the area within about 10 miles of the plant. Any radiation reaching the US should be negligible and produce no effects. There's a lot of space and dilution between us and Japan, along with decay of the radioactive material.
Hopes this helps understand the situation.
Molly's addendum: Japan, you are strange and unique and beautiful, and I love you! I'm so sorry for all that has happened to you lately. I also applaud the infrastructure and engineering you had in place beforehand, which kept these disasters from being much, much worse. You'll pull through this just fine, because you're awesome that way. Much love, your buddies on the other edge of the Ring of Fire.
Update, added 3/17/11: I asked Dad what kind of dose readings we're actually seeing, there and here. His answer...
Reliable dose readings at and around the plants are hard to find. I don't think TEPCO (who operates the plants) is doing a very good job reporting these, nor is the regulatory group in Japan.
I've seen the following numbers:
150 mrem/hr at 20 km
3000 mrem between Units 2 and 3
40,000 mrem beside Unit 3, then later falling to 1190 mrem, then 60 mrem
10,000 mrem inside Unit 4
75 and 34 mrem/hr at the plant gates
Remember that the average person gets about 600 mrem/yr from background. So some of these are very high doses and one would not want to spend very long in that vicinity. That's why they are recommending evacuation around the plants.
What might we see here? Depends on how serious the accident becomes. We might see a slight increase in the background here--we might pick up a few extra mrem. But that's like the dose you'd get flying from Seattle to Chicago.
The trouble with radiation is that it's too easy to measure. You can measure very small amounts--like one atom disintegrating. Many airborne toxins aren't measurable until they reach concentrations of 5-10% of a lethal dose! If radiation were this way, we couldn't measure it until it reached 3000 mrem or so.
We will probably know more what to expect in a few days, but I wouldn't worry about a very large increase in the background dose.