As part of the response to the reactor accident a map of radiation levels in Japan has been made, it can be seen at the following web site.
Also TEPCO have stated that they think that unit one did melt down and that it had a water leak, it is important to understand that the reactor may have melted the fuel in the past. By now as the fuel has had so long to decay it will have cooled down. I expect that the fuel will have turned back into a solid by now. I do not think that this change in our understanding of the state of unit one will change the degree of threat which the reactor now poses to the public, I suspect that the core melt occurred a long time ago back in March 2011. By now the radioactivity level in the fuel has gone down a lot so it is making less heat per cubic meter.
I think that the core melting progressed to a later stage than three mile island did, from what I know of the three mile island accident the core did not melt through the bottom of the pressure vessel. On the other hand this accident is no where near as bad as Chernobyl. At Chernobyl the core did melt but that was at a late stage in the accident, the majority of the fuel in the Chernobyl core formed a lava flow which dropped into the basement below the reactor.
I think that the vast majority of the release of radioactivity from the Chernobyl accident was caused by the steam explosion which occurred at time zero in that accident, the graphite fire increased the amount of iodine and other volatiles which were released. At Fukushima a series of explosions occurred, these were not steam explosions inside the containment or the reactor, instead they were hydrogen air explosions which occurred in the reactor building which is above the containment. While the hydrogen / air explosions damaged the buildings they did not scatter large amounts of nuclear fuel over the site in the same way as occurred at Chernobyl.
One thing which could have mitigated the problem with the hydrogen air explosions would have been hydrogen recombiners, there are two types. The active type try to ignite the hydrogen in a controlled way while the passive type use a palladium metal catalyst which allows the hydrogen to be converted by the oxygen of the air back into water.
If you imagine that a melt down is a engine fire in a car, then three mile island was an engine fire and Fukushima was a more serious engine fire. While Chernobyl was like driving your car into a tree, after the front end of the car was caved in it also caught fire.
On a lighter note I have seen a interesting film about a Japanese woman who has altered the wiring of a furby. Please note that I take no responsibility for any death, destruction, chaos, injury, feeling of self disgust, court case or other adverse outcome which may result from any attempt you make to alter any electronic device. Electronic device includes but is not limited to cute fur covered talking toys sold as furbys.