John Quiggin makes an interesting argument that war with Iraq is becoming less likely:

Suppose that this [UN inspectors not finding anything] continues until 26 January. By then, hundreds of sites will have been investigated, the best US intelligence will have been tested out, and the key Iraqi scientists will have been interviewed. If nothing has turned up, I can't see how Blix's report can possibly provide Bush with a casus belli. And by then, it will be too late to go back to the omissions in the declaration.

I think he's made a couple of unreliable assumptions. Regarding the last sentence I've quoted, I don't think going back to a report that was only a few months old is such an impossibility. Bush went back a decade to find evidence that Saddam was willing to use weapons of mass destruction. If anyone questions the delay, he can point to the claims that have already been made about the inadequacy of the Iraqi declaration, say he wanted to let the UN process run its course before making any moves, and if worse comes to worst point out that there's no point in launching the war before deployment to the Gulf (which is still in progress) was complete (implying support for Powell's "when you pick a fight, make sure you'll win" doctrine rather than Rumsfeld's "assume the best-case scenario" doctrine).

Second, I think the very fact that Blix's report doesn't provide any evidence (and I'm cynically inclined to say it won't regardless of what weapons Saddam actually has) could be made into a causus belli, with the added bonus of showing that the UN (and by extension world opinion) is irrelevant and need not be coddled. Bush has been so unequivocal in asserting that the US has ironclad intelligence on Saddam's weapons that he won't be able to back down from it, and the aspersions he's been casting on the inspectors' integrity and skill will further tip the balance. If Blix doesn't find anything, that just shows how sneaky Saddam is (and hence how much he's in breach of the inspections agreement) and/or how incompetent and yellow-bellied the UN is. Tony Blair will vouch for the US's secret intelligence, and the Anglo-American forces will roll.