Since the Japanese Government has been less than forthcoming about the radioactive contamination from the TEPCO Dai Ichi reactor disaster, and since TEPCO has been somewhat coy about the issue, here's a graphic that details measurements by the public in the affected areas.
Red means 5+ microsieverts per hour. Yellow-Green 1, Yellow 1.5 to 2.
These levels are not dangerous themselves, though long-term exposure would cause just-detectable increases in cancer rates. But they're indicative of where the cesium has gone to, and that, if ingested, really does pose definite increases in cancer.
TLDR version: do not eat any green leafy vegetables from the red area for 120 years, unless the topsoil's been replaced. Yellow - not in the next 30 years. Blues are safe, the rest, not for a few years, and make sure everything's been checked.