The Green Zone attacks were, symbolically at least, a sign that forces hostile to the United States are still able to strike at the American nerve center and seat of government power in the capital of Iraq.
http://www.iht.com/articles/2008/04/07/africa/07iraq.php
For a country purported to have won WWII practically single-handedly, the US (at $100,000 a minute) is being defeated by what is presented as a rabble of fanatical "hicks from the sticks". It is (at the very least) an indication that the interests backing this war can see no profit in bringing it to a conclusion.
Stumble It!