Surprisingly I have never had a need to use the double click event in ActionScript 3.0 until this morning. I presumed it would work exactly like the single click event so at first I just used this:

item.addEventListener(MouseEvent.DOUBLE_CLICK, listener);

But nothing happened! A little look at the docs revealed why, you just need to add this line:

item.doubleClickEnabled = true;
item.addEventListener(MouseEvent.DOUBLE_CLICK, listener);

Simple.