LiveLeak.com - Video of Islamic State of Iraq Shooting Down an American Chinook Helicopter in Al-Karma
(The images you see if you click there show an US helicopter being shot down. If this disturbs you, don't click. Other than that, this video shows no graphic violence, which is far less than any Hollywood movie depicting the war in Vietnam. I guess in twenty years, we will see the movies showing how the Iraq war really was and everybody is going to say "Never again" and forget it when the government warns of a new danger, a new Emmanuel Goldstein)
As to the quote:
Military officials are growing increasingly concerned that Iraqi insurgents are successfully adapting their tactics to be more effective against American aircraft.
A CH-47 Chinook is a 40 year old unarmored helicopter. I guess you could shoot it down with anything that is bigger than a small caliber weapon, a starting or landing Chinook could be downed with a AK-47, some skill and some luck.