Presumably Khalid Sheik Mohammed, the disco-dancing, skirt-chasing "muslim fundamentalist" who was on al-Jazeera last month describing how he organized the 9-11 attacks.
Or maybe he was let go on purpose, to see if he would go to Iraq and stay. Maybe he is what Rumsfeld was referring to when Rumsfeld said there was "bulletproof" evidence that senior Qaeda leaders were being harbored in Baghdad.