Here's a long, but good article that gives a good history of the administration's motivations for war in a pretty objective manner.

Mentioned in the article is a letter signed by the likes of Rumsfeld and Wolfowitz. I believe that this letter was a product of The Project for the New American Century.

Draw your own conclusions from this, but the war with Iraq was a long time coming. September 11th was merely a catalyst/excuse for PNAC, many of whose members are senior Administration officials now, to begin enacting the Iraq portion of their doctrine. Although I feel there are other motivations for this war, I would say that PNAC's beliefs in the need for regime change in Iraq is one of them.