Probable answer:
...these super-high-yield explosives probably being used in many, if not most, of the various suicide and car bombings in Iraq, but these particular explosives are ones used in the triggering process for nuclear weapons.
The Nelson Report is breaking this story and Josh Marshall spreads the word:
What also emerges in the Nelson Report is that the Defense Department has been trying to keep this secret for some time, even going so far as to order the new Iraqi government not to inform the IAEA that the materials had gone missing.
[update: Josh Marshall with more as promised]
The upshot seems to be that major media as early as tomorrow morning will be "On the Story". Marshall concludes with this thunderclap and potential lightning bolt:
It's a story that really brings together the adminstration's two cardinal sins: dishonesty and incompetence.
And what other stories are they trying to push back until after November 2nd?