One interesting rationalization for war that I have often heard is that Saddam, armed with nukes, would then proceed to invade Saudi Arabia and grab control of all the oil in the known universe.
Note that during the Cold War, our conventional military stationed in Europe was enough to deter a massive conventional invasion by nuclear-armed Russia. So the argument that we would be powerless to defend the middle east against a nuclear Saddam is flawed.
There is an inconsistency between this argument and the "Clash of Civilizations" idea, which posits that America is the chief villain to terrorists because of our freedom, which threatens their repressive interpretations of Islam. But Saddam is a socialist, unloved by the fanatic OBL crowd. The rhetoric on the recent supposed-OBL tape was clear on this point - referring to the Ba'ath party as "infidels".
Were Saddam to invade Saudi Arabia, Iraq would be at war with Islam. War on their brothers is one thing that Arabs are very good at - but it wouldn't be just Syria, Jordan, and Egypt - Iran and Pakistan would also enter the fray.
And what if our forces were deployed in defense of Saudi Arabia against Saddam? America might even become seen as the protector of Islam...
UPDATE: Tacitus comments below that we weren't seen as protectors of Islam during the Gulf War. True, but the difference is that Saddam invaded Kuwait, not Saudi Arabia. Since Saudi Arabia is the epicenter of Islam (and Kuwait is utterly insignificant), the two scenarios are worlds apart.
Targeting the holy peninsula for invasion is tantamount to expressing a desire for domination of Islam. Done by a socialist, it is intolerable to the radical fundies. America's stationing of troops alone has caused immense outcry and is one of OBLs stated rationales fo rtergeting America for jihad - that rationale is multiplied a thousand fold in the event of invasion from (socialist) nuclear Iraq.