It is not their M.O. to give warnings.
Maybe in the past it was not their MO but now that we are at war with al Qaeda I think it would be unwise to dismiss these chilling threats. We also have Arafat talking about a large explosion, we have our government issuing several alerts over the last couple of weeks, we have pills being distributed to citizens living near nuclear plants.
I do not think they are bluffing. I don't doubt for a minute that they are planning something bigger than 9/11. They pulled off a devastating sneak attack once and I pray we can stop them before the next one.
Not when they have something real in the works. They'd strike first for maximum shock value. The threats and bluster mean that they don't have much left to work with. I'd guess that, the more extravagant the threats, the less of al-Qaeda is actually in functioning condition. When they get to "we will exterminate every American on the face of the earth 12 times over, beginning next Tuesday, so be very afraid," that means that they're down to two crazy Arabs and a fax machine.