Correct me if I'm wrong, but in terms of fatalities today is so far the worst day for US military since the start of Iraq invasion. For many, especially on American left, the downing of helicopter could be that long-awaited "Mogadisciu Moment", after which US public's demand for "bringing boys home" would prove to be irresistible.
However, I don't think that this incident would result in the repeat of Somalian scenario.
First of all, unlike slaughter in Mogadisciu ten years ago, large number of US fatalities won't be much of a shock to American public. Nor would downing of US helicopter look that unexpected – guerrillas were trying to pull similar stunt for months, they only got lucky this time. And triple digit number of US soldiers killed before today did prepare US public for some unpleasant truths about fighting wars.
Furthermore, unlike in Somalia, USA simply can't afford to quit Iraq for all kinds of military, strategic and political reasons. Even if some ultra-lefty "peacenik" becomes US President, US soldiers are still going to remain in Iraq and continue to die in incidents like this one.
Instead of Mogadisciu, better analogies should made with Russian experiences in Chechnya. One day rebels managed to down Russian helicopter and kill between 80-120 soldiers. This is at least ten times more than today, but Russians nevertheless stayed in that quagmire. For them, just like Americans in Iraq, staying in Chechnya is least bad of many bad options.