Bottom line: Until people get inside the reactor, we don't know how much stuff melted. But there clearly has been melting for cesium and iodine to be released. Melting of the fuel rods is a very bad thing.

http://en.wikipedia....Configuration.jpg from http://en.wikipedia....e_Island_accident

Radioactive material release

Once the first line of containment is breached during a reactor plant accident, there is a possibility that the fuel or the fission products held inside can be released into the environment. Although the zirconium fuel cladding has been breached in other nuclear reactors without generating a release to the environment, at TMI-2 operators permitted fission products to leave the other containment barriers in a matter of minutes.[citation needed] However, since very little of the fission products released were solids at room temperature, very little radiological contamination was reported in the environment. No significant level of radiation was attributed to the TMI-2 accident outside of the TMI-2 facility. Noble gases made up the bulk of the release of radioactive materials from TMI-2, with the next most abundant element being iodine.

Within hours of the accident the United States Environmental Protection Agency (EPA) began daily sampling of the environment at the three stations closest to the plant. By April 1, continuous monitoring at 11 stations was established and was expanded to 31 stations two days later. An inter-agency analysis concluded that the accident did not raise radioactivity far enough above background levels to cause even one additional cancer death among the people in the area. The EPA found no contamination in water, soil, sediment or plant samples.[28]

Researchers at nearby Dickinson College, which had radiation monitoring equipment sensitive enough to detect Chinese atmospheric atomic weapons testing, collected soil samples from the area for the ensuing two weeks and detected no elevated levels of radioactivity, except after rainfalls (likely due to natural radon plate out, not the accident).[29] Also, white-tailed deer tongues harvested over 50 mi (80 km) from the reactor subsequent to the accident were found to have significantly higher levels of Cs-137 than in deer in the counties immediately surrounding the power plant. Even then, the elevated levels were still below those seen in deer in other parts of the country during the height of atmospheric weapons testing.[30] Had there been elevated releases of radioactivity, increased levels of Iodine-131 and Cesium-137 would have been expected to be detected in cattle and goat's milk samples. Yet elevated levels were not found.[31]


(Emphasis added.)

If we're to believe the reports, radioactive cesium and iodine have already been detected in Japan, so one could infer that what has happened to the core is worse than what happened at TMI. That's just an inference at this point, though.

I don't expect the steel reactor vessel to fail, but the folks who designed and who are working on the reactor obviously didn't expect to have so much trouble keeping it filled with water, either...

In the near term, the longer this goes on without them being able to flood the core, the more risk there is of large radioactivity releases.

Cheers,
Scott.