Addenum to last post by Charley-
IAEA monitors say Japanese authorities believe that the cores have been damaged in Daiichi units 1, 2 and 3. This would explain the radioactive iodine 131. Best case scenario- venting.

A rather complete summary, dated 22 March for the last few additions:

http://www.iaea.org/newscenter/news/tsunamiupdate01.html