One expert speculates that the unit 2 core has melted through the floor

Excerpt from the Guardian -

"The warning follows an analysis by a leading US expert of radiation levels at the plant. Readings from reactor two at the site have been made public by the Japanese authorities and Tepco, the utility that operates it.

Richard Lahey, who was head of safety research for boiling-water reactors at General Electric when the company installed the units at Fukushima, told the Guardian workers at the site appeared to have "lost the race" to save the reactor, but said there was no danger of a Chernobyl-style catastrophe." Finally, Richard Lahey states "The indications we have, from the reactor to radiation readings and the materials they are seeing, suggest that the core has melted through the bottom of the pressure vessel in unit two..."

But indications from the bottom of the reactor haven't changed much.  The known damage from the fuel storage pools and other sources can account for current radiation readings.  The temperature sensors are still functioning at the bottom of the reactor vessel at unit 2.  If the corium melted through, we should not be getting any temperature data from the bottom of the reactor.  I think we need to see further confirmation before anyone can claim that the core has melted completely through the pressure vessel. 

This development is still speculation, and has not been verified by TEPCO.