I tend to think of the Flex arrays and collections with a Java mindset, which is bad, as they don't follow the same patterns. Consequently I have a bad habit of either coercing an ArrayCollection to an Array implicitly, or explicitly and being mystified when they aren't a clean cast. They are different of course: Array and ArrayCollection documentation from the AS3 docs. The Array is used compositionally in the ArrayCollection and is a wrapper around the Array that implements the List and Collection functionality. The Array can be pulled from the collection via:

import mx.collections.ArrayCollection;
var arrCol:ArrayCollection = new ArrayCollection();
var arr:Array = arrCol.source;