http://subtech.g.hatena.ne.jp/secondlife/20070211/1171120066

var source:String = [
'<?xml version="1.0" encoding="UTF-8"?>',
'<ItemLookupResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2005-10-05">',
	'<OperationRequest>',
	'</OperationRequest>',
	'<Items>',
		'<test>hoge</test>',
	'</Items>',
'</ItemLookupResponse>',
].join("");

var x:XML = new XML(source);
var aws:Namespace  = new Namespace("http://webservices.amazon.com/AWSECommerceService/2005-10-05");
trace("x.aws::Items = " + x.aws::Items);


trace("x.aws::Items.aws::test = " + x.aws::Items.aws::test)

default xml namespace = aws;
trace("default = " + x.Items);

とかいう話、では、ないの、かしら、勘違いの可能性大（寝ぼけてる）