Luftansa and Alitalia flights to or from Tokyo are being diverted elsewhere, such as Osaka. Fortunately, so far, no radiation has been found on their planes.

The bulk of the foreigners in Japan are closest to Tokyo, so if the situation deteriorates and other carriers do the same and more foreigners want to leave the country, that is going to be a massive headache.

The Tokyo domestic airport (Haneda) is already full of Japanese trying to escape to western parts of Japan, so adding an extra influx of foeigners at Haneda trying to also go to those same cities will result in a huge bottleneck.

Trains are certainly an option to Osaka and elsewhere, but due to the electricity cutbacks, I believe that trains heading west are alredy running on reduced schedules. Just getting to the airport can be a challenge due to the rotating blackouts and reduced train schedules.