People who left Tokyo and got on an airplane have already exposed themselves to more than 3 X the radiation than they would have had they stayed in town. Some numbers to keep things in perspective:
One hour in Tokyo today: .809 microsieverts
One hour in Cornwall, UK (lots of granite) normally: 1.2 microsieverts
One hour in an airplane: 3 microsieverts
Chest X-Ray: 100-200 microsieverts
One hour at the front gate of Fukushima, 8:30AM today: 8,217 microsieverts (8.2 millisieverts)
Hospital CT Scan: 12,000-25,000 microsieverts (12-15 millisieverts)
We are monitoring the situation continuously. The US Ambassador is staying in Tokyo. Another friend said that the French have left only because they could not find someone to surrender to.