The Fleisher comment I gave you was 5/7/03, which I just realized was after the invasion, and thus less relevant. Here are some uses of the phrase "imminent threat" from before the invasion:
Scott McClellan, 2/10/03, saying "This is about imminent threat."
Dan Bartlett, 1/26/03, saying "Of course he is" when asked if Saddam was an imminent threat.
To have Dan Bartlett, Scott McClellan, and Ari Fleisher -- all professional communicators -- make that same "mistake" suggests it was an idea the administration was dancing around and trying to imply for a long time.
Could you please post the context and source for this? If I recall correctly, this was McClellan talking about Turkey invading Iraq, not us invading Iraq.