Bradley Lewis picks up on an interesting thread on Mirror of Justice, and expands on it in a thoughtful and compelling manner. I'd like to add my comments as well.

Referring to WMD as the "main" justification for war, even if true (a claim with which I would take issue), is irrelevant. The relevant question is whether, absent WMD as an argument, there was sufficient justification for war. Look at the actual authorization for war passed by Congress in 2002. In complex matters, we write contracts because, with time, memories fade, interests change, and commitments waver. HR 144 lists at least 11 reasons for authorizing force against Iraq:

a) Repeated violations of the 1991 ceasefire
b) Interfering with efforts of UN weapons inspectors
c) Continuing to possess and develop WMD
d) Aiding and abetting terrorist organizations
e) Repression of its citizenry
f) Refusing to release prisoners wrongly detained
g) Failing to return property wrongly seized from Kuwait
h) Attempting to assassinate former President Bush in 1993
i) Firing on US and Coalition forces who were enforcing UN Resolutions
j) Harboring al Qaeda terrorists
k) Failure to comply with UN Resolutions 678, 687, 688, 949, and others

There were other reasons as well, broader strategic reasons like restoring international peace and security in the Persian Gulf region, propagating democracy, etc.

Now according to Section 2309 of the Catechism of the Catholic Church, there are four criteria for a just war:

2309 The strict conditions for legitimate defense by military force require rigorous consideration. the gravity of such a decision makes it subject to rigorous conditions of moral legitimacy. At one and the same time:
- the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
- all other means of putting an end to it must have been shown to be impractical or ineffective;
- there must be serious prospects of success;
- the use of arms must not produce evils and disorders graver than the evil to be eliminated. the power of modem means of destruction weighs very heavily in evaluating this condition.

So let's take these four criteria, one at a time.
Criterion 1
Lasting: Saddam had been in power and causing misery and death for three decades, and his sons appeared to be next in line to continue their father's "work". And if we're going to use hindsight to throw out the WMD rationale, how about including, then, the corruption of the UN through the Oil-For-Food scandal, where a rogue state was able to use illegal cash to bribe the very body that was supposed to be monitoring its compliance.
Grave: If you look at the list above, even if you strip out items b) and c) having to do with WMD, you still have a pretty compelling set of reasons. Especially the hundreds of thousands of Iraqi citizens who had been systematically killed and oppressed.
Certain: Again, even setting aside the WMD issues (which seemed certain to everyone at the time), there is no doubt that Saddam would continue his nefarious behavior.

Criterion 2
Impractical: Saddam was a murderous tyrant. What means, other than war, have been used successfully to stop such people?
Ineffective: We had tried economic sanctions, military no-fly zones, embargoes, diplomatic isolation, targeted strikes, inspection regimes, etc. etc. etc. We had tried to get Saddam to comply for more than a decade, and to no avail.

Criterion 3
Prospects for success: No one ever doubted that the Coalition could win a war. And with good reason, in retrospect...

Criterion 4
Proportionality: There can be no debate on this criterion; the data simply don't support any other conclusion. As MOJ points out, according to some (non-Bush-friendly) sources, there were fewer than 20,000 civilian deaths in Iraq in 18 months. That was an average year for Saddam's regime, according to some authorities. In addition, the technological capabilities of the US made proportionality more feasible. The ability to target precision weapons sharply reverses a millenium-long trend toward more collateral casualties, and broadens the scope of a just war.

Conclusion
Based on this analysis, the war appears to be just. Moreover, I have never read an analysis of the Iraq War which directly applied the Church's just war criteria and come to the conclusion that the war was unjust. (Please send me a link if you have seen one.) Most people simply believe the war has to be unjust, and assume the Church's teaching has to support this position.

But an assertion is not an argument. And I believe the most negative thing you can say at this point is that the war may be unjust, but the question is inconclusive.