I don’t think a Sunni/Shia clash is inevitable at all. Most sectarian conflicts are borne of a sense of real oppression and hopelessness. If a middle class can rise in Iraq, and the standard of living of all can be improved, the Iraqi people will have a lot to lose in a civil war. If an equitable oil deal can be reached that brings money to everyone, and a federalist system developed that gives each region enough autonomy to believe that they can manage their own affairs while still benefiting from living in a greater Iraq, I think Iraq can come back from the brink.
This is what the surge is hoping to accomplish. The federal government in Iraq sucks, so the military’s strategy has been to rebuild a civil society from the bottom up and empower local governments. So far, that seems to be working. Economic activity in Iraq is booming. New businesses are starting up every day. People who fled the country are coming back. Shops are re-opening. And better yet, local leaders are starting to reach out to other groups and form economic and security alliances that expand the scope of peaceful areas.
This in turn will hopefully cause the people at the grassroots level to put more pressure on higher-level politicians to get their collective acts together. This also is happening, and a number of the original benchmarks for reconciliation have now been met and more are being negotiated.
The longer the U.S. can keep Iraq from blowing apart, the greater the likelihood that it will find a way to glue itself back together again.