The UN has a specific departement designed to help rebuild countries. It is known as the 'trusteeship council', basically a country is held 'in trust' by the UN and is rebuilt with the help of member nations. It has the 5 permanent members of the Security council at it's head. I would suggest that this organization be the one to rebuild Iraq for the sheer fact that while the US and the UK are the main factors in the war on Iraq, I am not sure how willing they would be to spend billions of dollars to rebuild a country on the other side of the globe (correct me if i'm wrong). An international body would also mean that the interests of the peoples of Iraq are held to be primary, so the no country (France, Germany, Uk, Russia, US, etc) takes advantage of the situation. The council has been dormant since 1994, but is still considered one of the main branches of the UN. for more info:
http://www.un.org/documents/tc.htm