Now, before the Son of Gulf War got kicked off, there were alot of appeals to the UN to do something. The members of the UN were opposed to a war in Iraq, for what ever reasons. But one member and a handful of allies tried to force it through; a vocal minority hoping to outweigh the majority. They failed. So, they ignored the UN's attempts at arbitration (what the UN is for) and did what they intended to anyway.
Now, this war has lead to the capture of Saddam Hussein, who we can say with decent confidence, is a war criminal. Mind you, he does remain innocent until proven guilty, both under US law and under UN regulations for such tribunals. Now, the UN wants to take this potential war criminal, and try him for his crimes. So does the US, but the US wants to do it on its own terms, claiming that "The people of Iraq are the victims, and should make the judgement." Did the citizens of Germany try the Nazi officers in the Nuremberg trials? No; it was an international tribunal.
I'm going to take one guess at the US's motivation for this continued challenge to the UN. Bush, who while governor of Texas presided over the most executions for any governor, wants Saddam to swing and do the gallows dance. A trial administered by the UN cannot deliver the death penalty.
Regardless of his motivation, what is the UN going to do about it? The first test of the UN's power and resolve happened before the Son of Gulf War. The UN failed that test. Now what? The only power the UN has comes from the cooperation of its members. This means that it is up to the member nations to choose some sort of stance on the issue of Saddam's trial. Is the UN going to bring International standards to these proceedings, or is it going to allow one member free reign to ignore the UN yet again.
The United States is the most powerful member of the UN, and organizations like The Project for a New American Century make it clear that members of the administration desire to flex that power unilaterally. And with Saddam the second test of this stance is being raised. Are the member nations of the UN going to force the US to abide by the UN's rules? Remember, the UN itself has no power- its only ability to act comes from its member nations.
If Saddam is tried and found guilty (which he must be- or the war continues to lack clear justification) by an Amer-iraq tribunal a second nail will be hammered into the coffin of international cooperation. By the time the next election rolls around, the UN will be defunct in all meaningful ways.
Now, I don't love the UN. I'm wary of meta-governments on a world scale. But the UN isn't and rarely attempts to be a world government. It's a nation-club, where countries can get problems solved with cooperation from other countries. An admirable goal. So, while I would like to see some reform in the running of the UN, I don't think the organization should be scrapped. I admit, there's not alot the international community can do to express its distaste. Organized tarriff increases perhaps? Downright embargo? Dangerous ground to be on with the US's recent behavior. But at the same time, we either cooperate or we don't- there can't be any halfway. So perhaps the UN expels the US- when someone doesn't play by the club rules, that's what you usually do. A token action, admittedly, but one that shows the international feeling.
Some information on what the tribunal needs to succeed: http://www.tompaine.com/feature2.cfm/ID/9620