I followed the link in the TEPCO (Nuclear) tweet from about an hour ago, and the landing page was the email notice to the media on May 4, 2013.
The notice says TEPCO happened on a piece of debris cleared from the Reactor 3 operating floor as it was being loaded on the special remote-controlled truck that carries highly radioactive debris. The piece was emitting 540 millisieverts/hour radiation.
Also in the notice (my translation),
At 1:45PM today [May 4], access was restricted around the truck that was carrying the debris.
There is no effect on the workers or the debris removal work.
There is no significant change in the monitoring post measurements.
In the most recent notice on the debris, about 30 minutes ago,
The high radiation debris that was in the remote-controlled truck has been moved to the temporary storage areas for highly radioactive debris, which is located on the south side of the Reactor 3 building.
Within 5 days or so, using the remote-controlled truck for the high radiation debris, we will move it to the Solid Waste Storage on the north side of the Anti-Seismic Building. We will pay attention to radiation exposure when we move the debris.
Moving the highly radioactive debris from one temporary storage to another. Then what? No end-state envisioned.
Has TEPCO been mapping the debris as they remove from the Reactor 3 operating floor, and recording the radioactivity of each pieces? Probably not. I wish they did. It might have given some clues on the explosion - how it happened, or what kind of explosion it was.