Here's a link to the Register's editorial last week on the new National Intelligence Estimate on Iran. If I do say so myself, it includes a pretty good succinct description of just what an NIE is, which implies why all of them including this one, are conditional even when the summary sounds certain, to wit:
"A National Intelligence Estimate represents the official consensus view of the government's 16 intelligence agencies, prepared by the National Intelligence Council under the direction of the Director of National Intelligence. Because of the necessarily incomplete and ambiguous nature of intelligence, especially about a society effectively closed to outsiders, NIEs are couched in terms like "judge," "assess," estimate, "probably" and "likely," further modified by terms like "high," moderate" or "low" levels of confidence. NIEs are supposed to be objective – free of bias for or against current policy – but different analysts, all of whom have personal preconceptions, often look at the same set of facts and come to different conclusions."
However accurate this one is -- I think its significance is that it is an official rebuke to Cheney and to some extent Bush, and I'm reasonably sure Cheney at least tried to suppress it -- it would be incredibly stupid to bomb Iran, however emotionally satisfying it might be to some people.