As I noted earlier, radiation levels at Japan's Fukushima power station have reached 400 millisieverts (mSv) per hour. The chart below (posted by @ gakuranman) helps put this figure into perspective. A typical chest X-ray, for instance, involves exposure of about 0.05 mSv, while a stomach X-ray involves 0.6 mSv. The annual amount of natural radiation is roughly 2 mSv; the current limit for nuclear industry employees is 20 mSv per year.
According to the World Nuclear Association, 100 mSv a year is the lowest level at which any increase in cancer is clearly evident; absorption of more than 500 mSv can depress white blood-cell levels. A single dose of 1,000 mSv causes radiation sickness such as nausea and vomiting; a single dose of 5,000 mSv would kill about half of those receiving it within a month.
Following the Chernobyl disaster in 1986, those exposed to levels greater than 350 mSv were relocated. In Japan, the government has ordered everyone within 30 kilometres of the danger zone to stay indoors, and has imposed a no-fly zone around the power station. The US navy's 7th Fleet, stationed 100 miles offshore, has retreated after 17 crew members were treated for radiation exposure.