I'm far from a conspiracy theorist but your idea is plausible for your stated reason and others. As much as many of you would argue, our foreign military policy is greatly dictated by world opinion. As an example I use Desert Storm. World opinion turned against us almost instantaneously after the images of our forces strafing and killing retreating Iraqi soldiers on the road to Basra. Almost immediately after that Bush ordered the cease fire and subsequent end to the war. Now we have to go back and finish what should have been taken care of 10 yrs ago.
As far as the current war is concerned, Bush has stated that our aim is to kill Osama Bin Laden, Al Qaeda, and his terrorist network. For now, world opinion is still on our side. The minute we announce (God willing) that Osama is dead world opinion will start to sway against the war (Not to mention our lovely Liberals within the US will start calling for an end). They will view any additional attacks on our part as nothing more than retribution on our part. Knowing this would you announce to the world that Osama has been found and killed? I most certainly wouldn't until I have accomplished everything I set out to do (the annihilation of Al Qaeda and all other threats to our country). I'm not saying its true but it most certainly is a plausible theory.