My take are that there are 4 parts to such a scenario;

1- What will happen on the ground in Iraq/the region after the pull-out
2- How will the pull-out be executed and who will order it
3- What impact this will have on the US government
4- How will the history of the Iraq invasion be subsequently interpreted  (by Iraqis,  foreign Islamists and Americans)

All are extremely important; it is not evident that the inmediate impact on Iraq itself will necesarily be the most important result (for instance, the impact on Vietnam and Somalia were not the most important results of our pull-out from those countries), though in this case it will be made far more likely because of oil.

My guesses (and they are only that; there are still far too many moving pieces) are that

1- The civil war will continue to get bloodier, and the present ethnic cleansing /death squad activity will continue to increase. The iraqi government's writ will not extend past the Green Zone (arguably, it already doenst).  The Maliki government will fall, undermined from within by Al-Sadr and maybe SCIRI. Either one of those will proclaim an Islamic Republic in all or part of Iraq. Holding new elections is not likely to be possible given both the security situation, and the fact that elections will most likely produce an even more radical and polarized government. The likeliest outcome, therefore, is that upon losing support the Maliki government will declare martial law but will be ineffective even at that, given it's precarious control over the army/police.

Subsequently Iraq will splinter into 3 areas, like Bosnia did after it's own civil war.  Of those only the Kurdish area could by any measure be considered a success. The Shia region will fall under the direct influence of Iran, who will call the shots from Tehran. The Sunni area will become a lot poorer (no oil), and probably become a failed state.  The price of oil, and the instability of oil prices will both rise significantly.

2- The pull-out will be ordered by the present Administration, starting in the next 6 months or so, and will be presented as a 'tranfer of sovergnity' to the Maliki government.  This will be resisted and resented by the (increasingly fewer) architects of the war who remain in policy-setting positions.  The main drive for it will come from the Republican party itself, still smarting from the loss of the Congress and Senate, and increasingly looking to the next presidential election rather that defering to the lame-duck presidency.  Significant numbers of Republicans, particularly senators, will side with the democrats on key votes.  The pull-out itself will start out gradually and attempt to retain order while talking-up the 'transfer of power' to the 'sovereign government' until that falls when Al-Sadr pulls his support and Maliki declares martial law.  Political statements from within the Administration will increasingly shift to blaming Iraqis themselves in the closing days of the Bush administration. 

The appearance of splinter factions or independent candidacies within the Republican Party (possibly Ross Perot-like candidacy, or possibly a Fundamentalist christian faction) are very likely. Such splits will be less likely amongst Democrats (who still vividly remember Ralph Nader's key role in the election of GWB), but they are also possible

3- Harder to call, as it depends on what government arises after the end of the lame-duck Bush presidency. 

Almost certainly it will be a Democratic one, and in either case it will be 'realist' rather than neo-con, whether it's a Democratic administration or a post-Bush Republicans.  The only possible event that could save a neocon candidate will be the succesfull execution of a large scale terrorist attack on US soil or a US stike on Iran (more so the former). Even that may or may not be able to create a government that resembles the Bush administration.  Economic crisis caused by surging oil prices in the wake of the collapse of Iraq will tend to make this less likely.

4- This is likely to mirror the reactions to the last elective war we lost, the Vietnam war.  The mainstream consensus will be to see the elective war on Iraq as a mistake of historic proportions.  The fringe view will be that things were coming along swimingly, that we were winning in Iraq, and that we only lost because particular groups (the media, the democrats, traitors within, various conspiracies, etc.) forced America to pull out of a war it was winning.  This revisionism will likely remain a far-fringe view for at least a generation.

In Iraq, it is likley that they natinalistic "we-defeated-the-empire" views will lend further support to the more extreme parties, particularly the religious ones in the Iran-dominated Shia areas and to a lesser extent in the Sunni areas.  The Kur