How imminent was the threat?

How This Old Brit Sees It points us to an interesting report on the NBC site:

NBC News has learned that U.S. and British authorities had a significant disagreement over when to move in on the suspects in the alleged plot to bring down trans-Atlantic airliners bound for the United States.

A senior British official knowledgeable about the case said British police were planning to continue to run surveillance for at least another week to try to obtain more evidence, while American officials pressured them to arrest the suspects sooner. The official spoke on condition of anonymity due to the sensitivity of the case.

In contrast to previous reports, the official suggested an attack was not imminent, saying the suspects had not yet purchased any airline tickets. In fact, some did not even have passports.