It's not exactly shocking, really; ever since the war was declared 'over' the Sunni insurgents have been targeting the Shia parts of the country in order to stir up a civil war.
The other effect is scaring people away from joining the Iraqi security, government, etc, services - essentially making it impossible for any form of independent government to even try to exist ( one which is not dependent on US troops for 'security', for example). As a result the US either has to commit to a longer and longer term stay (getting stuck deeper into the quagmire and strengthening the political and popular opposition at home), or back out early (humiliating themselves across the world and handing a victory to guerilla forces and possibly islamic militants).
And if there is a civil war sparked off, then the US is truly ****ed - they'd be caught in a 2, 3, or n-way battle (Sunni vs Shia, Sunni vs Shia with Kurdish bid for independce, the last one with Turkey and Iran wading in....or so on); they couldn't really take sides, because the weakest side - the minority - is the Sunni community that has pretty much rejected them.
Of course, you already knew that anyway...