[ActionScript側]

import mx.data.components.WebServiceConnector;

var res:Function = function (evt:Object) {
    trace(evt.target.results);
};
var wsConn:WebServiceConnector = new WebServiceConnector();

var req = new Object();
req.SubscriptionId  = '########';
req.AWSAccessKeyId  = '########';
req.Request = new Array();
var ResponseGroup = new Object();

ResponseGroup.Keywords = 'iPod';
ResponseGroup.SearchIndex = 'Electronics';
ResponseGroup.ResponseGroup = ['Small', 'Images', 'ItemAttributes'];
req.Request.push(ResponseGroup);

wsConn.addEventListener("result", res);
wsConn.WSDLURL = "http://webservices.amazon.com/AWSECommerceService/AWSECommerceService.wsdl";
wsConn.operation = "ItemSearch";
wsConn.params = req;
wsConn.suppressInvalidCalls = false;

wsConn.trigger();

<SOAP-ENV:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
<SOAP-ENV:Body>
  <ItemSearch xmlns="http://webservices.amazon.com/AWSECommerceService/2005-10-05">
  <SubscriptionId>[object Object]</SubscriptionId>
  <AWSAccessKeyId>10B4NDTY1XGR8PFK1K82</AWSAccessKeyId>
  <AssociateTag>10B4NDTY1XGR8PFK1K82</AssociateTag>
  <XMLEscaping />
  <Validate />
  <Shared>
    <Actor />
    <Artist />
      ・
      ・
      ・
    <TextStream />
    <Title />
  </Shared>
  <Request xmlns="" />
  </ItemSearch>
  </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

なんかオブジェクトに突っ込んだ逆順に上から入れられてる？？
Request要素とか空だし...

ボク根本的に間違えてる気がする．
ご指摘ありましたらお願いします．m(_ _)m

追記）
XMLを直接指定して投げる分には取得できた…
んー，こうじゃないきがするなぁ…

var xml = '<SOAP-ENV:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">' + "\n";
xml += '<SOAP-ENV:Body>' + "\n";
xml += '  <ItemSearch xmlns="http://webservices.amazon.com/AWSECommerceService/2005-10-05">' + "\n";
xml += '  <SubscriptionId>########</SubscriptionId>' + "\n";
xml += '  <AWSAccessKeyId>########</AWSAccessKeyId>' + "\n";
xml += '  <Request>' + "\n";
xml += '    <Keywords>iPod</Keywords>' + "\n";
xml += '    <ResponseGroup>Small</ResponseGroup>' + "\n";
xml += '    <ResponseGroup>Images</ResponseGroup>' + "\n";
xml += '    <SearchIndex>Electronics</SearchIndex>' + "\n";
xml += '  </Request>' + "\n";
xml += '  </ItemSearch>' + "\n";
xml += '  </SOAP-ENV:Body>' + "\n";
xml += '</SOAP-ENV:Envelope>' + "\n";


var reqXML = new XML(xml);
reqXML.ignoreWhite = true;
var resXML = new XML();

resXML.onLoad = function() {
  trace(this);
};

reqXML.sendAndLoad('http://soap.amazon.co.jp/onca/soap?Service=AWSECommerceService', resXML, 'POST');

追記）
んー，RESTの方がいいかなぁ...
http://webservices.amazon.co.jp/onca/xml?Service=AWSECommerceService&SubscriptionId=10B4NDTY1XGR8PFK1K82&Version=2005-10-05&Operation=SimilarityLookup&ResponseGroup=Small&ItemId=4797327030

参考）
・ITmediaエンタープライズ | 特集 | WebサービスをAmazonで知るECS 4.0でアフィリエイト
・最新WebサービスAPIエクスプロ-ラ ~Amazon、はてな、Google、Yahoo! 4大Webサービス完全攻略