“It’s a sad fact of military aviation that helicopters flying in combat are accidents waiting to happen.” They are slow, they fly low, and they have lots of external vulnerable targets. They can be brought down by small-arms fire. Most of the downings that the Pentagon is quick to assert did not result from hostile fire actually turn out indeed to have been shot down. The role of copters in the upcoming ‘surge’ remains to be seen… (Time)

Also: 

Copter Crashes Suggest Shift in Iraqi Tactics: “American officials say the streak strongly suggests that insurgents have adapted their tactics and are now putting more effort into shooting down the aircraft….Some aspects of the recent crashes indicate that insurgents have become smarter about anticipating American flight patterns and finding ways to use old weapons to down helicopters, according to military and witness reports. The aircraft, many of which are equipped with sophisticated antimissile technology, still can be vulnerable to more conventional weapons fired from the ground.” (New York Times )