You said -- "Perhaps they're just waiting for the right sort of 'target rich' envrironment. Something like, say, hundreds of thousands of people out in the streets of a large number of major cities all at the same time?"
They also may be looking for a "connecting event" in the Middle East. In other words, this sort of thing may be sprung on the U.S. at the same time that several countries invade Israel. It might be designed to keep the "enemy" (in their eyes) busy at home, while they do the "elimination of Israel" thing that they are so intent on doing.
Regards,
Star Traveler
IMHO, one thing that bin Laden and the crazy Iranian and others have in common is the return of the Mahdi, the 12th imam. They would save 'the big one' to fit into that prophecy somehow.