Al Qaeda's Internet communications system has suddenly gone dark to American intelligence after the leak of Osama bin Laden's September 11 speech inadvertently disclosed the fact that we had penetrated the enemy's system.

Just a pointer: this story is also being discussed in another thread centered around an article by Joby Warrick in the Washington Post.

RE: Qaeda Goes Dark After a U.S. Slip - October 9, 2007 - The New York Sun