I'm speculating, so please consider my words with a large grain of salt.
I am fairly convinced that the Bush administration is in possession of some very hard evidence that Iraq not only has weapons of mass destruction and has been positioning them for use, but also has provable ties to active terrorist organizations, probably but not necessarily including Al Qaeda. The ties may even involve safe harboring of many terrorists in Iraq, which would partially explain why the U.S. has been quietly surrounding Iraq with troops and security agreements (such as with Jordan) for more than a year.
Iraq may very well be Afghanistan II, with the toppling of a hostile regime and terrorist hunt scenario. Except in this case, the stakes are raised, because Saddam is much craftier than Mullah Omar, and the Iraqi military has much greater resources than the Taliban (remember them?) ever dreamed of.
I think it is extremely unlikely that Bush will invade Iraq without the hard evidence he needs to put egg on the face of the U.N. and the Democrats, both of whom he despises and both of whom will look like idiots once the truth comes out.
Why hold back the evidence, rather than using it to build a groundswell of domestic and international support? I think there are two main reasons, as well as several lesser ones:
Reason 1: Military and intelligence security. Saddam knows we know about his weapons and evil intentions. However, he doesn't know exactly what or how much we know. If he did know, he would quietly take steps to both eliminate the evidence (or move it out of sight) and eliminate those assets that revealed it. He could have an entire weapons program's staff and their families tortured and slaughtered without batting an eye, and has a reputation for exactly that. As Sun Tzu and every other great military leader has pointed out again and again, deception is the greatest weapon of war.
Reason 2: Politics. The invasion of Kuwait by Iraq was an obvious and egregious enough act to build support for a coalition to drive Saddam out. Revelations of nasty weapons, terrorist ties and war plans are not as dramatic, and, even in the wake of 9-11, make for a harder sell to a skeptical domestic and international constituency. Naysayers who would oppose war no matter what (including the now heavily Liberal and Marxist Democratic Party, which used to be much more hawkish) have been expending their energy fighting a straw man crafted by the vague and unsupported claims made so far by the Bush administration. I find it extremely unlikely that Bush is lying about Iraq, but I notice that he has not been at all forthcoming about details. This is apparently deliberate. Those foolish enough to jump on the bandwagon against him without any real basis to do so will be utterly discredited when he lays his cards on the table. Cries of "foul" about being kept in the dark will be trumped by the legitimate need for operational security, and the naysayers will be neutralized politically. This, of course, applies to the Democrats, but especially to the U.N., the relevance of which Bush is openly challenging.
There are many other good reasons why Bush should wait until the eleventh hour before tipping his hand, including the generally mercurial nature of the American public, but I think the two reasons above are the big ones.
Consider that after Iraq is conquered, a government benevolent to the U.S. will be installed, oil will be sold in abundance to pay off Iraqi debts and rebuild the country (castrating OPEC -- and we're working on Venezuela, btw -- and, of course, the U.S. economy will flourish with all this cheap oil coming in = Bush + Republicans win again in 2004), a security agreement with the U.S. will be implemented that allows us to use Iraq as a huge, excellently positioned military base (no more begging the Saudis for permission, etc.), and puts U.S. forces in a much stronger position to pursue and eliminate both terrorist organizations and the regimes that support them, and invading Iraq is a no-brainer.
I could be wrong, but everything about the way the Bush administration is acting supports my suspicions to the tee. Everyone who has underestimated the shrewdness and wisdom of George W. Bush in the past has come to regret it. He's extremely clever, and, in my opinion, a modern Abraham Lincoln, who was also underestimated by his opponents and used it to stunning advantage.
I like to call his leadership style "strategery". ;^)