You should really have a look at this very comprehensive and informative entry on MutantFrog.com:
There is an awful lot of panic and speculation regarding the situation at the Fukushima #1 (Daiichi) Nuclear Power Plant and in particular its possible effects on the Tokyo Metro area. What is really going on?
Short answer – things seem to be pretty safe for now, but there is still a possibility of danger if things don’t go well.
A nuclear scientist on MetaFilter mentioned the following, based on what is currently known.
… while I would certainly be keeping a close eye on the situation if I were in Tokyo (!), I would not yet be very worried about health effects (or finding concrete bunkers) given the information currently available.
Another explosion this morning spewed a large amount of radioactive material into the air. The actual vessel containing the core has been exposed. NYT The minister says “we are now talking about levels that are harmful to human health but we ask you to embrace this information calmly.”
Radiation around the reactor is now equal to three years ordinary exposure every hour. The winds have shifted and a thermal plume is expected to impact Tokyo by this evening. As of now (14:45 Tuesday), radiation levels in Tokyo are 22 times above normal. It is said that “this level will not immediately affect health.”
It is recommended that all people outside return indoors immediately unless absolutely necessary – and then use a mask. When returning brush off clothes and leave them outside. Do not drink tap water or have any food that has been outside (though food and water seem to be unavailable in my neighborhood shops). Close and tape all windows and turn off any heating or other ventilation system. Limit skin exposure after washing with soap and water.
Rain is expected shortly and any contact should be avoided due to radioactive contamination. The same is true for the snowfall forecast for tonight and tomorrow.