I see two basic possible outcomes here:
1) If US Stays, two things happen. First, the insurgency is slower to pick up speed, but after the coalition fractures and UN troubles cause other troop pull outs, it picks up pace. Secondly, 9/11 exactly is butterflied away, but there's very likely to be some other direct attack on the US by Islamisist terrorists.
2) If the US takes Saddam down and then pulls out, the resulting power vacuum pulls in first Iran and Turkey, and shortly after an Arab coalition (Saudi-Syria). After an initially short sharp three way war and a genocide of the Kurds, we have a Mexican Standoff between the three. Turkey looses any chance of EU membership due to the Kurdish problem, and turns its energies towards Central Asia (the 'Stans). The result here re the US is probably even more hostility than OTL or 1.