Caveat to this Post (updated information)
Not sure if the advice below is any good. There are two issues: the stuff below is a throat-wash for colds, so I do not know if you absorb any iodine; second, I do not know if it is the type of potassium-iodide that stays in your system. I do know that I do not trust the Japanese government to tell us any of the truth, or to send out the good pills soon enough to matter. My native-wife says I should calm down. Wrong approach. I raised my voice and told her I saw the hash-job they did in Kobe when half of those people did not need to die. Whoops.
Make sure you have potassium-iodide: ヨウ化カリウム (youka-kariumu).
You take it prophylactically, so that your thyroids cannot take up any (radioactive) iodine from the environment, thus protecting yourself from the most commonly lethal side-effect of radiation exposure. Besides staying in and not drinking irradiated water, this one prevention will dramatically improve your odds of staying healthy. I am not going to wait for the Japanese bureaucracy to suck-wind up and down the hierarchy before handing it out, and neither should you.
March 13 addendum
Here is the English link to the Tokyo power company, where you may find information on the rolling blackouts: the link has not been loading, however. Here's information about how they have lied in the past. Likewise the JR East Kanto train delay page in English is 'under maintainance'; the subways do not have useful English pages; here is something in Japanese. Fill a bunch of bottles, and your bathtub, and go shopping for food. Shelves are starting to thin. If you have a camp stove or table-top stove, get some fuel. This will probably come to nothing, and if it does you will just use this stuff in future for daily life, right?
March 14 addendum
You don't need me to summarize the news, and are probably aware that another reactor building went 'boom'. If you live here and don't know much about Japanese culture yet, let me put it to you simply: you won't get the truth from the authorities until it is too late to do you any good. The Japanese don't believe them either. If the winds come onshore, bunker down. Use foreign news services, but avoid the exploitative mass-market American ones. I have not decided to run yet, partly because it may be overreaction, but mainly because it's doubtful I could get my family flights, or to Narita/Haneda by train or car. Consider your own options, including waiting it out. Your main risk, if there is fall-out, is what you ingest, if you manage to stay inside.