There is only one thing that both the Sunnis and the Shias can agree on. They both want the United States to leave. Here's the deal. We'll leave, but only if these two warring factions sign a ceasefire.
Now let the party begin. The Iraqis can celebrate saying that they defeated the Americans. Meanwhile Bush can lie and say that a stable democracy has been established and essentially America has won. The Iraqis only have to stick to the ceasefire for a short period of time and then they can decide if they want to split the country along sectarian lines. Sure ceasefires aren't perfect, but if the Israelis and Palestinians can temporarily decrease the violence, so can the Iraqis.
We start leaving and bring in the UN Blue Helmets to help keep the peace. Bring in every Arabic speaker they have and tell the troops that their primary job is to CONGRATULATE THE IRAQIS. Instead of Iraqis firing guns at each other, they turn their guns to the sky in celebration.
The Iraqi government will then declare that after a short time (6 monthes or so) there will be a referendum vote to decide whether the country should be divided. This could also temporarily quell the violence. After living in relative peace for a while, people in Iraq might actually find that they enjoy it!
This plan is far from perfect, but it will save a lot of lives, and it's a hell of a lot better than an escalation.