Are we to believe that the consensus of the world’s intelligence agencies was formed by everyone relying on a single INC-affiliated engineer’s report? I think I would believe that Hussein hid the WMDs in a secret cave in Tibet before I would believe that.

I agree with Mr. Coleman that the Iraqi National Congress (INC) was not the force that convinced the Bush administration (or anyone else, for that matter) that Iraq had WMDs. (I am not saying that Saddam did have them, though).
I think that Chalabi told the neocons (and from them, people in the Bush administration) what they already believed, and what they wanted to hear in order to push his own agenda (namely, power for himself and his friends in Iraq). In other words, he provided those who believed in WMDs and in removing Saddam supporting information/ disinformation to help them reinforce their claims. But the idea that Chalabi is responsibe for the belief in WMDs is laughable (although the INC may be the source for some of the more specific claims, that is).
Ultimately, Chalabi cannot be blamed (or credited, epending on your point of view) for getting the US into Iraq with disinformation; we would have gone in whether he helped the administration make the case for war or not. To the extent that he was giving us disinformation, the import is that he is untrustworthy, and that he can be blamed for some of the mistakes in the planning of this war, to the extent that he influenced our strategy in a negative way (e.g. making us assume our welcome in Iraq would be warmer than it was).