There are now a number of excellent websites that give detailed analyses of the events at Fukushima.
There is an excellent general article on Fukushima by Richard Wilson on public health aspects as well as some detail on the incidents at the power plant. Richard Wilson has written extensively on risk-benefit analysis and his catalogue of disasters puts this event very much closer to Three Mile Island than to Chernobyl. There are also some interesting comments on the consequences of evacuation enhancing the risk of cancers. Finally there are some pointed remarks on the news media reporting nuclear incidents. The ABC ia a good example of selective reporting with one interview from an Australian school teacher living in Tokyo.
A thorough explanation of the events, as currently understood, is on the AREVA website. The analysis has detailed diagrams covering both the reactors and the spent fuel storage. Most concern and uncertainty appears to be directed at the spent fuel stored in the pool at Unit 4. This is because all the nuclear fuel was transferred some three months before the earthquake into the pool so there is a much greater thermal load in Unit 4 than in the pools by the other reactors. The term “criticality” has been used in some reports on the state of the spent fuel pools. This does not mean a nuclear explosion but rather the possible sustained release of radioactive material.
Finally there is an extensive set of articles on Wikipedia that covers most aspects. Their links to definitions of technical terms enables you to drill down into many aspects of nuclear reactor operation and safety.
In summary, the power plants buildings are mostly intact unlike Chernobyl. This means that the fission products released in steam will fall to ground close to the plant.
The main radioactive contributions outside the plant will be carried by wind but with decreasing activity and none or little local contamination of soil.
Great care has been exercised monitoring radiation exposure of plant operators.
For the surrounding populace, the stress of leaving their homes may cause more harm than the radiation risks. There will probably be no need to create a protective land zone around the plant.
The remaining and important issues are how well the company (TEPCO), regulators, politicians and the media managed and reported the event.
Nuclear power is at present the only viable candidate for baseload power supply if fossil fuel power generation is to be reduced.