And 3. What weapon(s) will be used in the attack? Biological (anthrax, small pox, etc.)? A conventional (fission) nuke? A stolen (or bought) thermonulear fusion bomb from the Russians? A "dirty" bomb? VX or sarin gas?
To attempt to answer your two main questions, I'd suspect that an eventual WMD attack on our shores is very high. However, Saddam Hussein doesn't necessarily have to be behind such an attack. There are multitudes of Islamic terrorist orgs (aside from al Qaeda), and a lot of them are well financed. So the attack can originate from almost anywhere in the radical Islamic world.
When is the attack most likely to occur, you ask? If Saddam is behind it, expect it very shortly (within a couple of months). If it's an Al Qaeda operation, the answer is .....when we least expect it.
The strategic corollary to this is that, when an attack does come, we will not be able to identify the attacker. How can we then know who to retaliate against? (After all, in the midst of crisis with country A, country B might decide to attack the U.S. covertly, figuring that the U.S. will retaliate against A, incorrectly assuming that A was the guilty party. We would then have been an unwitting patsy of country B, and our actions would, in fact, encourage the future use of biological weapons against us, because B's secret attack would have succeeded in its goals.)
Moreover, Iraq and everybody else will know that, in the absence of identification of an attacker, retaliation isn't a possibility. Therefore, the former guarantee of retaliation for the use of WMD has ceased to be a deterrent; the attacker just must be careful to deploy the weapon covertly and untraceably.
With the failure of deterrence as a strategic discipline, the only option remaining to us is pre-emption: stopping the development of any WMD which might be a threat to us, before it is used. That's what we're doing with Iraq right now.
By the way, an attack could even originate from some place outside the Islamic world. We do have other enemies who might be sufficiently emboldened to do something like that if they thought they would not be caught.