CASI [Campaign Against Sanctions on Iraq] press release dated 7th January 2003:

A "strictly confidential" UN document, written to assist with UN contingency planning in the event of war with Iraq, predicts high civilian injuries, an extension of the existing nutritional crisis, and "the outbreak of diseases in epidemic if not pandemic proportions."

the document is titled [LIKELY HUMANITARIAN SCENARIOS] and was apparently mentioned for the first time in this Times article (23rd of December 2002):

THE United Nations is making secret contingency plans for a war that would halt all Iraqi oil production, “seriously degrade” the country’s electricity system, provoke civil unrest and create 900,000 refugees, The Times has learnt.

CASI says that it has obtained a draft of this document through a UN source who has authorized the publication of parts of the document. With all the talk about human shields and anti-war protests none of the "human shields" is thinking of a "B" plan. And from what little I have heard most international agencies including UNHCR are saying they are not really prepared or don't have enough funds in case of a "humanitarian emergency" in Iraq, and even if the funds were available, getting the goods to where they are needed is also a problem. It makes pretty grim reading.
There is one term which I have not seen before. [IDPs] Internally Displaced Persons. The report estimates the number of IDPs at 2million. Refugees to bordering countries at around 900,000 to Iran and 50,000 to Saudi Arabia, they would be " from Baghdad and the Centre Governorates" (paragraph 17). I guess the western desert makes Jordan and Syria a bit too difficult to reach but Turkey would probably also see a lot of refugees (well, they have prepared their tents within the Iraqi border there anyway). A more recent article on the UNHCR site says that the expected number of refugees is around 600,000 :

The United Nations High Commissioner for Refugees, Ruud Lubbers, said Tuesday between 500,000 and 600,000 Iraqis are likely to become refugees in a U.S.-led military action in Iraq.
Lubbers told a news conference in Geneva that half of the Iraqi refugees are expected to flee to Iran while the other half are likely to head to Turkey in the north.

makes more sense than going to Saudi Arabia. But there is one little fact that the UN document mentioned above does not state, this is from the UNHCR:

Lubbers said a military conflict in Iraq will also produce a large number of internally displaced people in Iraq fleeing the war but, as in the case of the war in Afghanistan, these people would not be covered by UNCHR since they are not classified as refugees under international law.

so what happens to the 2 million IDPs ?
back to [LIKELY HUMANITARIAN SCENARIOS]
The report summarizes the scenarios by splitting them into two stages Emergency and Protracted Humanitarian Requirements. Here is the list for emergency requirements:

- Bridging, material handling and transport. - Food and necessities for some 5.4 million people.
- Health supplies to treat injuries for approximately 100,000.
- Health supplies to treat the highly vulnerable for up to 1.23 million.
- Health supplies to cater for the ongoing needs of 5.4 million.
- Nutrition supplies for 0.54 million.
- Water treatment equipment for 5.4 million.
- Chemicals and consumables for 5.4 million.
- Sanitation materials and chemicals.
- Total range of services for 2 million IDPs, some of whom may well become refugees. The number that may eventually be in this category cannot be assessed with any confidence.
- Emergency shelter for 1.4 million.
- Family reunion facilities for unaccompanied minors.
- Facilities for 100,000 Iraqi refugees in neighbouring countries.
- Mine Action activities, (demining, UXO clearance, mine awareness).

you notice it doesn't make any mention of possible doomsday scenario if one of the sides uses "unconventional" weapons. I guess if that happens it would be out of everybody's control, rather not think about it.
Here are all the links, take a look at it:
LIKELY HUMANITARIAN SCENARIOS [html version]
LIKELY HUMANITARIAN SCENARIOS [scanned PDF of original]
LIKELY HUMANITARIAN SCENARIOS [4-page PDF booklet]