Just read the Carnegie piece, and I judge it less than compelling. I think that Pollack's argument can be taken down by undermining one or both of two assertions: 1) Saddam will acquire nuclear weapons if we don't have a permanently and aggressively active inspections program (or an invasion); 2) if Saddam acquires nuclear weapons he will use them, or the threat of their use, to inflict significant human suffering on his neighbors and the world at large.
On point 1 there is room to make a case, but the authors fail to do so. Pollack marshals some evidence to make these supporting claims: Iraq has uranium deposits sufficient to meet its needs; Iraq has been able to secure the materials for its nuclear program despite the sanctions; and Iraq has the know-how to eventually succeed in producing nuclear weapons on a clandestine basis (secret at least against standard intelligence and satellite reconnaissance). The Carnegie piece does not meaningfully counter the point about Iraq's ability to acquire materials despite sanctions -- it resorts to simple assertion. It also makes the standard unreassuring assumption that the aggressive inspections (and U2 reconnaissance) regime can be maintained indefinitely. But we know that wasn't true in the past, and we can easily observe that restarting the process has involved great expense and terrible (and potentially unsustainable) tensions within the international community.
On point 2, I think they just punt. The problem here is that Saddam with nuclear weapons is far less deterrable and far more dangerous than Saddam without. Pollack does make a compelling case that Saddam's past behavior has at times been reckless. He goes further to sketch out the mechanisms by which Saddam comes to an unreliable world view that encourages his aggressiveness. The authors quibble with the meaning of some of the examples Pollack uses to make these points, but they totally miss the larger issue.
I think a big reason these points haven't been well debated is that Bush has clouded the issues by trying to tie the case for invasion to the al Qaeda links. Those are certainly not well substantiated by the publicly available evidence. So the debate tends to muddle up around that issue. (Similar to what was noted in an earlier post about Bush's willingness to use misleading arguments, I think the administration has made a tactical decision not to justify invasion the same way Pollack does. I don't fully understand their reasoning.)
