I don't actually know of a UN report on livestock that specifically states that livestock emissions outweigh all forms of transport combined. This is inferred from the data on greenhouse gas emissions by sector.
The livestock's long shadow is an excellent report that tells you most of the facts you need (Part IV, page 82) for the livestock industry sources in general.
The IPCC probably does not tie together the meat consumption with greenhouse gas directly, which is why we have to explain it to people :) The things to look for are the Agriculture sector, and the Forestry or Land Use Change sector.
The primary gas from the livestock industry is methane (CH4), which cows and sheep produce as part of their natural digestive system (the average cow produces 400 litres of methane per day). The rest comes from manure treatment and some component is the effect grazing animals have on the land itself.
In countries that import their livestock, or are primarily grain fed, the industry has less impact from land use. The IPCC states the emissions from agriculture compared to other sectors in an excellent starting point which is a document for policy makers. In section 2 page 5 of the document there is a pie graph showing the sources of CO2-e from different sector. Agriculture is listed as contributing 13.5%, transport is 13.1%. In Australia the livestock industry makes up 71% of the agriculture sector (agriculture includes plant and animal industries). The figures for Australia are from the Initial Report to the UN 2005 data (submitted last month to the UN).
This is not the full story though, as in countries like Australia and Brazil, most livestock (cattle) are fed from pastures, and not grain fed until their last 100 days. In order to produce grazing pastures, large tracts of land are cleared, which makes up about 90% of the deforestation as well (90% of 17.4% which is 15.66%). So when you add up the cost of the industry, which combines the direct from the animals, and the secondary from the land practice of land clearing, it makes up about 30% in Australia. No livestock, no land clearing.
This is why in the options for agriculture in the Garnaut submission it states there are three options. 1: do nothing (very bad) 2: stop land clearing. 3: stop it all. The stop land clearing is not very practical, as it increases demand on grain supplies, and would be unviable in Australia to do so. Land clearing is much cheaper, which is why it must be included in emissions trading - so that the cost is bourne by the industry, and it would not be cheap any more.
The specifics to Australia come from the "Tripple Bottom Line" report from the CSIRO and University of Sydney, which is referenced in the submission.
The Bruce Poon report provides a very good summary of the CSIRO report, and what it means.
BTW: The submission was written with the objective of highlighting that the agriculture sector has been ignored in discussions to date, and all of the focus has been on electricity generation and vehicle fuel. Livestock emissions are less than electricity, but more than vehicles.
In discussions with persons of importance such as a Director of Climate Change government department, it will be important to state and agree that the #1 problem is coal fired power stations. The second highest emission sector is livestock, followed by transport. Without removing coal fired power stations world wide, global warming will become catastrophic. The removal of livestock will not solve global warming, but it will certainly delay the effects for about 40 years (est to be revised.. probably often). Also, the cost of reducing emissions needs to be fairly distributed to all industries. If agriculture is excluded, then the carbon emissions allowances for electricity and transport will have to be 30% higher than they need to be. Say that it costs $100 to remove the desired portion of the CO2-e. To meet that cost, the electricity bill and fuel bill must go up $100, since 30% of the emissions (from livestock) are not included, therefore subsidised. If livestock is included, then you can choose not to eat meat, and your electricity and fuel bill only go up by $70.00. The livestock industry have stated they will join emissions trading at some time in the future ("as soon as practicable") but that sort of language can give excuses that "they are not ready yet" for a long time, delaying vital action. On a personal note, it makes me sad that my fuel bill will be subsidising the livestock industry.
To put it into perspective, James Hansen report on how much CO2 states that we should aim for 350ppm (parts per million) of CO2-e. Currently it is 385ppm. Removing the livestock industry would remove 25ppm, which would get us half way to the safe level from todays measurements. Note that the IPCC recommendation is > 400ppm, which is based on 2005-2006 data, and hopefully will be revised.
I hope this helps, sorry it is so long. Sometimes simple questions have not-so-simple answers.