A TPM reader theorizes on the events leading up to the new Iran NIE:
There are, oversimplifying, two threads going around, one that the 'Iran doesn't have an active program' preliminary finding was circulating in the Administration in late '06 (Hersh, etc), and the other that Bush was told about the upcoming finding in August by McConnell, after which he changed his characterization so as not to be so obviously lying about the nature of the threat (all the while still intentionally leaving a grossly misleading impression).
The two threads can be reconciled. The basis for the findings had, indeed, been circulating beginning in late 2006, and ever since. One has to assume that Cheney and his forces marshaled full fire on those findings, and successfully suppressed them, preventing their release. That effort, however, eventually failed, probably due to intelligence and Pentagon unwillingness to take the fall for another war.
What happened in early August was not that Bush learned of the findings, but that McConnell informed him that the NIE containing the findings would be released. Those on the side of releasing them (which had to have included Gates) simply won the battle, and either faced down Cheney, threatened to resign if they lost or utilized whatever other strategy required. It was not the discovery of the underlying truth of the findings that caused the change in rhetoric (becoming more vague on Iran's nuclear status, but more bellicose on Iran generally). It was the realization that the NIE would become public.
Fixating on what the President was told in August, while important, misses the larger power play almost certainly underway for many months before then. Late Update
: The Horse's Mouth has more on the big questions yet to be answered
about the run-up to the release of the new NIE.