All 6 reactors at Fukushima Dai-Ichi are now in various states of meltdown and/or systemic cooling/containment failure, the two to watch most closely are Unit 2 (probable pooling-floor vessel breach) and Unit 3, due to the plutonium/uranium MOX fuel (this is one of only 2 MOX fueled reactors in Japan). All but Unit 6 are the weakest type (Mark 1) of GE containment systems. Unit 6 is Mark 2.
If 1 or more of the units has a vessel floor breach,the world will get large varying dispersement doses of radiation. The super-reactive pile of fuel pellets will congeal into a nuclear fire-fueled mass, and bore into the containment building floor, then continue boring into the earth, until it hits the water table. At this point, the water is instantly turned into a super-charged radioactive steam jet that will blow upwards with such force that it shoot through the borehole, then the building and up into the atmosphere at jet stream level.
This will be dispersed world wide, and the radiation levels will be extreme. A nuclear steam kettle, if you will.
The true nightmare is if the floor breach occurs in Unit 3 (the MOX plutonium/uranium fueled one), as not only will the force of meltdown be greater but the deadly plutonium will increase the radioactivity by exponential levels. And if any of this plutonium, even at infinitesimal levels, gets into a person, they are a dead man/woman/child walking.