I'm not sure I really take the word of a Pakistani journalist seriously. I don't know why they'd be waiting for a "massacre of Muslims" to act if they really have a cell here capable of taking action. That's not to say we shouldn't take the threat of terrorism seriously. We should. It's just to say we shouldn't take the bloviating of Al Qaeda vermin and their stooges in the Islamic press overly seriously.
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.