@benm
However, It would be nice to see some facts to back up the "Don't worry," since there is a possibility of real danger should the situation get worse.
As far as i've seen, the information we get is all they can feasible know at the moment. There is just no way to see what is going on inside the core of a damaged reactor, you only see temperature and pressure readings.
The 'real' danger is also impossible to predict, since it will be something between 'nothing to worry about' and 'complete meltdown'. In the latter scenario there will be release of radioactive materials into the environment, but nothing like the chernobyl disaster.
The current situation is that the main nuclear reaction that normally provides power has been stopped (it was stopped before the tsunami). The current activity is due to remaining fission products in the fuel bundle, not from the actual fission of uranium. Depending on the age of the fuel elements this still produces quite a lot of heat, and that is the problem: Cooling is required to prevent the fuel bundle from melting due to its own activity.
Without, the fuel rods will melt, pool up on the bottom of the inner vessel, and possible melt through. Theoretically this melt can continue though the secondary container and the concrete floor. After that, it will end up in the ground, polluting ground water, releasing radioactive vapours - the china syndrome (or in japan, it would be whatever is on the other sde of the planet from their perspective).