With regard to preemption of attacks and terror, it doesn't matter whether a dirty nuke "is a WMD" or some other kind of weapon. I am interested in preemption, not in Preventing Things That Are Technically WMDs But Allowing Things That Aren't.
With regard to the legalistic issue of whether Iraq was in violation of relevant UN resolutions, it doesn't matter. The UN resolutions banned Hussein from having certain things. The list was not "things that are technically WMD", after all, one thing on that listed was: unmanned air vehicles. (Which is also Not A WMD by itself.)
Except that the whole point of Stanley Kurtz's article was to attempt to use the existence of radioactive materials which could possibly be incorporated into a dirty bomb as legalistic evidence that Saddam did in fact possess WMDs.
If you want to argue that there were other valid and sufficient reasons to invade Iraq and overthrow Saddam, that's fine. Do it. But if you are being defensive about the lack (so far) of any smoking-gun evidence that Saddam possessed WMDs, this argument just ain't gonna fly. No one who isn't already convinced on other grounds that the Iraqi war was justified is going to be convinced by the fact that some radiactive materials were uncovered and that such materials could conceivably have been inserted in radiological bombs. That simply doesn't translate into a Weapon of Mass Destruction by any meaningful interpretation of that phrase.
Frankly, most biological and chemical weapons also make lousy weapons of mass destruction, as we saw with the anthrax-in-envelopes attacks. The anthrax killed several people because we were caught by surprise. For anthrax or some other pathogen to truly cause mass destruction would require an extremely virulent and weaponized version along with an effective delivery system. The same is true of chemical weapons. But at least the potential is there. Natural smallpox has killed millions of people in the past. Chemical weapons have been used in past wars and have killed thousands. There's a modicum of plausibility in including them under the rubric of "Weapons of Mass Destruction".
The real WMDs remain explosive nuclear devices. A suitcase nuke going off near a crowded sports stadium could instantly kill tens of thousands. That kind of threat is worth seriously worrying about. Reliable intelligence that such weapons were in the hands of terrorists or rogue regimes would be an excellent argument for pre-emptive action.