Soil sampling results will really tell the tale about what is going on near the reactors in Japan, and whether and when people can go back there.
Greenpeace notes that they’ve done some soil sampling 60 km from the reactors, but I don’t find the results yet. And results closer in should be very revealing.
One take-home message is that large particles settle faster than smaller ones. So, larger particles containing radionuclides will have settled nearest to the plant. I’m using “near” in relative fashion… near could mean hundreds of meters, or many kilometers. A lot will depend on weather conditions as well.