According to CNN.com:
"A new audiotaped message purported to be from al Qaeda leader Osama bin Laden warned Americans that plans for attacks in the United States were already under way.
"We have seen explosions in many European countries. As for similar operations taking place in America, it's only a question of time. They are under way, and you will hear about them soon," said the message, which was aired on Arabic-language network Al-Jazeera Thursday."
IF bin Laden does manage to carry out a terrorist strike in the U.S. in the near future, then this changes the face of the War on Terror. Depending on the nature of the terrorist act, that could mean the measures we have taken to this point were worthless or inadequate.
On the other hand, if bin Laden is bluffing, then the War on Terror continues as is.
I don't think bin Laden is bluffing. In poker terminology, bin Laden doesn't strike me as a bluffer. He may not win the hand, but he definitely THINKS he has a winning hand.