According to CNN: Defense Secretary Leon Panetta arrived in Baghdad for the ceremony to personally thank the U.S. troops who have served there, as well as Iraqi security forces.
All U.S. troops must be out of Iraq by the end of the month after Washington and Baghdad failed to agree on terms under which they could remain. The cable network includes the voices of military personnel who served there.
The Guardian has stories saying the withdrawal is "an act on a stage," and that it portends terrible things for world stability. Take your pick.
Fox has a less extensive report that emphasizes, oddly I think, that no "stray" shots were fired today.