Saddam was a public figure, we knew (more or less) where he was... Osama's a guerilla leader. Saddam had a nation we could declare war on, Osama has an increasingly underground organization spread thinner than paper throughout much of the world. Faced with two enemies the public equally hates, Bush picked the easier target.
I don't think Bush chose one over the other. I think part of the divide between supporters and detractors of the war in Iraq is that its detractors tend to view the "War on Terrorism" as a cynical, resource-burning propaganda piece akin to the "War on Drugs", whereas the supporters (of which I'm one) see it as a literal war, to which the habits and assumptions of peacetime diplomacy are as inappropriate as, say, trying to dissuade an armed rapist by asking him sympathetic questions about his unhappy childhood.
According to this view, both the welfare of the citizens of Iraq and the potential of arresting WMD development are components of a larger package. Other components include:
* Demonstrating to all regimes that might have a hand in terrorism that the United States, contrary to past appearances, has not become a paper tiger and will not countenance sponsors of terrorism. The exact relationship of Saddam Hussein to terrorism has yet to be determined (though I think it's pretty well established that he at least offered rewards to the families of Palestinian suicide bombers); but if there's no clear relationship, then that makes America all the more frightening, because it means America will act if it even thinks you're supporting terrorism.
* Shaming the proponents of theocratic fascism (because shame, not death, is what the terrorists fear) by creating, right in the heart of the Middle East and the cradle of civilization, a prosperous liberal democracy to serve as the Arabic world's counterpart to Israel. I don't deny that this is an ambitious goal and fraught with many risks. But if it succeeds, Iraq will be the model to which all other Middle Eastern nations (or more precisely, their citizens) aspire.
The invasion of Iraq may seem a cruel and brutish tactic to achieve these ends; particularly so, if one has the luxury of not comparing it to the casualties of any past wars. But it is a model of restraint compared to what will happen to the Middle East if the terrorists persist in their optimism and one or two more major attacks succeed in America.
I think it's pretty clear what happened... Saddam, a shifty opportunist, had disarmed long ago but was hedging his bets by resisting arms inspectors and avoiding providing evidence of either weapons or their destruction. Why? He wanted to be safe from both sets of enemies... the western world would be kept at bay because they couldn't prove he had WMD.... his closer neighbors and oppressed subjects would behave because they couldn't prove that he didn't.
That sounds plausible. Whatever the case may be, I suspect that the truth about the fate of Saddam's WMD will come out sooner or later. My own suspicion is that the WMD claims will be vindicated to some degree, but I've been wrong before.
w00t
~wolf01
[EDIT] BEFORE THEY KILL HIM!