Perhaps, but there is a sort of (very) twisted morality in the way bin Laden operates. He understood that Zarqawi's targeting of innocent Muslim civilians in Lebanon and the Beslan massacre of children did nothing to help his cause.
He has played by the (muslim) rules of offering a truce for those who accept Islam.
If there IS another sleeper cell in the US, there must be a reason that they are waiting.
Waiting for a "justification" seems to me as good a reason as any.
The sceaming over Haditha may have already put a cell in motion