Fukushima Nuclear plant explosion:

Calculations on the radiation levels:
Current reported dosage levels @ 1000*normal.
Normal radiation dosage @ 2.4 mSv/year
need 1~2 Sv dosage to start to feel nausea or vomiting -> time to that occurs is about 5 ~10 months assuming that you are *at* the power plant.

Dosage levels will probably drop in an inverse squared power law away from the power plant. Meaning we've got plenty of time to react.

Sources:

Radiation rate @ 1000 x normal: http://bit.ly/eGht2S
Radiation poisoning levels: http://bit.ly/bRheP6