In one of the last moments before the end, the passenger who remains the most composed through the ordeal (he makes the first phone call) raises the "bomb parts" over his head and crows that it is indeed fake. So yes, they had an inkling that the bomb wasn't real, but moved anyway.
Don't forget that the only verifiable info about anything that happened aboard [aside from the cockpit recordings] is what was cell phoned in; -- is there any indication that some one actually raised the "bomb parts" over his head and said that it is fake?