At the moment there are serious questions about the ability of the UN to assist in organising January's elections in Iraq. The UN withdrew after the bombing of their headquarters and death of Sergio Vieira de Mello last year, and will not return unless there is adequate security. As a result, they have only a skeleton staff in Iraq.
However, it turns out that there was a solution to this problem: a special security force solely to protect the UN, made up of troops from Muslim nations (who would hopefully be better able to negotiate with Iraqis and therefore far less likely to be shot at). The UN and the Saudis had lined up several countries to contribute troops, and the interim Iraqi government was on board - but the deal was nixed at the last minute by the Americans. Why? Because the troops "would have been controlled by the UN instead of by U.S. military officers who run the Multi-National Force in Iraq". A compromise deal where the troops would have been under Iraqi command was similarly rejected (I guess allowing the Iraqis to control foreign troops in their own country would have set a bad precedent...)