【作成環境】
　・WinXP Pro SP3
　・Adobe Flex 3.4.0.9271 (Adobe AIR 1.5.2.8870)

【実行環境】
　・WinXP Pro SP3
　・Adobe AIR 2.0.2.12610

【Main.as(AIR)】
　※関連していそうな部分は色を変えてあります。

package{
    …省略…
    //
    import my.Log;
    import my.FlexDefaultComponents;
    import my.AIRDefaultComponents;
    //========================================================================//
    public class Main extends WindowedApplication{
        //ユーザー名 ※通常は埋め込まない
        private var m_user:String = "********";
        //パスワード ※通常は埋め込まない
        private var m_pass:String = "********";
        //アプリケーション識別名
        private var m_source:String = "example.com-YT-2010062901";
        //デベロッパーキー
        private var m_dev_key:String = "********";
        //お気に入り登録したい動画のID
        private var m_video_id:String = "********";
        //認証時レスポンスデータの保存用
        private var m_auth_respons:Object = new Object();
        //ログ用
        private var m_log_tarea:TextArea;
        //--------------------------------------------------------------------//
        public function Main() {
            super();
            FlexDefaultComponents.stylesInit();
            AIRDefaultComponents.stylesInit();
            Log.trace("i Main");
            //
            this.layout = "absolute";
            this.setStyle("backgroundColor", 0xEFF1F1);
            this.addEventListener(FlexEvent.APPLICATION_COMPLETE, app_complete);
        }
        //--------------------------------------------------------------------//
        private function app_complete(a_event:FlexEvent):void{
            Log.trace("i app_complete:" + a_event);
            //
            var acbar:ApplicationControlBar = new ApplicationControlBar();
            acbar.dock = true;
            var label:Label = new Label();
            label.text = "YouTubeの動画をマイアカウントのお気に入りへ登録";
            acbar.addChild(label);
            this.addChild(acbar);
            //
            var root_layout:HBox = new HBox();
            root_layout.setStyle("paddingLeft", 8);
            root_layout.setStyle("paddingTop", 8);
            root_layout.setStyle("paddingRight", 8);
            root_layout.setStyle("paddingBottom", 8);
            root_layout.y = 8;
            root_layout.percentWidth = root_layout.percentHeight = 100;
            this.addChild(root_layout);
            //
            var left_layout:VBox = new VBox();
            left_layout.setStyle("paddingLeft", 8);
            left_layout.setStyle("paddingTop", 8);
            left_layout.setStyle("paddingRight", 8);
            left_layout.setStyle("paddingBottom", 8);
            root_layout.addChild(left_layout);
            //
            m_log_tarea = new TextArea();
            m_log_tarea.setStyle("fontFamily", "_typewriter");
            m_log_tarea.setStyle("fontSize", 11);
            m_log_tarea.editable = false;
            m_log_tarea.percentWidth = m_log_tarea.percentHeight = 100;
            m_log_tarea.text = '';
            root_layout.addChild(m_log_tarea);
            //
            var list_item:Array = [
              {label:"認証", type:"auth"}
            , {label:"お気に入り登録", type:"add_favorite"}
            , {label:"ログ消去", type:"eraselog"}
            ];
            var tbbar:ToggleButtonBar = new ToggleButtonBar();
            tbbar.direction = BoxDirection.VERTICAL;
            tbbar.buttonMode = true;
            tbbar.dataProvider = list_item;
            tbbar.addEventListener(FlexEvent.CREATION_COMPLETE, tbbar_complete);
            tbbar.addEventListener(ItemClickEvent.ITEM_CLICK, tbbar_item_click);
            left_layout.addChild(tbbar);
        }
        //--------------------------------------------------------------------//
        //ボタンバーのイベント関数
        //--------------------------------------------------------------------//
        private function tbbar_complete(a_event:FlexEvent):void{
            Log.trace("i tbbar_complete:" + a_event);
            //
            a_event.currentTarget.selectedIndex = -1;
        }
        private function tbbar_item_click(a_event:ItemClickEvent):void{
            Log.trace("i tbbar_item_click:" + a_event);
            //
            m_log_tarea.text += "[" + a_event.label + "]\n";
            switch (a_event.item.type) {
                case"auth":
                (function():void{
                //
                //「認証」ボタンが押された時
                //
                //中身のデータを作成
                var content:URLVariables = new URLVariables();
                content.Email   = encodeURI(m_user);
                content.Passwd  = encodeURI(m_pass);
                content.service = "youtube";
                content.source  = encodeURI(m_source);
                //リクエストデータを作成
                var req:URLRequest = new URLRequest(
                    "https://www.google.com/youtube/accounts/ClientLogin"
                );
                req.method = URLRequestMethod.POST;
                req.contentType = "application/x-www-form-urlencoded";
                req.data = content;
                //リクエスト
                var urlloader:URLLoader = new URLLoader();
                urlloader.addEventListener(IOErrorEvent.IO_ERROR, io_error);
                urlloader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, security_error);
                urlloader.addEventListener(Event.COMPLETE, auth_complete);
                urlloader.load(req);
                })();
                break;

                case"add_favorite":
                (function():void{
                //
                //「お気に入り登録」ボタンが押された時
                //
                //中身のデータを作成
                var content:String = 
                      '<?xml version="1.0" encoding="UTF-8"?>'
                    + '<entry xmlns="http://www.w3.org/2005/Atom">'
                    + '<id>' + m_video_id + '</id>'
                    + '</entry>'
                ;
                /*
                //↑こちらでもOK↓
                var content:XML =
                    <entry xmlns="http://www.w3.org/2005/Atom">
                        <id>{m_video_id}</id>
                    </entry>
                ;
                */
                //リクエストデータを作成
                var req:URLRequest = new URLRequest(
                    "http://gdata.youtube.com/feeds/api/users/default/favorites"
                );
                req.method = URLRequestMethod.POST;
                req.contentType = "application/atom+xml";
                req.data = content;
                req.requestHeaders.push(
                  new URLRequestHeader("Authorization", "GoogleLogin auth=" + m_auth_respons.Auth)
                , new URLRequestHeader("GData-Version", "2")
                , new URLRequestHeader("X-GData-Key", "key=" + m_dev_key)
                );
                //リクエスト
                var urlloader:SocketURLLoader = new SocketURLLoader();
                urlloader.addEventListener(IOErrorEvent.IO_ERROR, io_error);
                urlloader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, security_error);
                urlloader.addEventListener(Event.COMPLETE, add_favorite_complete);
                urlloader.load(req);
                })();
                break;

                case"eraselog":
                //
                //「ログ消去」ボタンが押された時
                //
                m_log_tarea.text = '';
                break;
            }
        }
        //--------------------------------------------------------------------//
        //URLLoaderのイベント関数
        //--------------------------------------------------------------------//
        private function auth_complete(a_event:Event):void{
            m_log_tarea.text += "i auth_complete" + "\n";
            //レスポンスデータ分解、分かりやすく保存しておく
            var respons:String = a_event.currentTarget.data.replace(/\n$/, "");
            for each(var key_val:String in respons.split("\n")) {
                m_auth_respons[key_val.split("=")[0]] = key_val.split("=")[1];
            }
            //確認
            for (var key:String in m_auth_respons) {
                m_log_tarea.text += key + "[" + m_auth_respons[key] + "]\n";
            }
        }
        //--------------------------------------------------------------------//
        private function add_favorite_complete(a_event:Event):void{
            m_log_tarea.text += "i add_favorite_complete" + "\n";
            //確認
            m_log_tarea.text += a_event.currentTarget.data + "\n";
        }
        //--------------------------------------------------------------------//
        //共通のイベント関数
        //--------------------------------------------------------------------//
        private function io_error(a_event:IOErrorEvent):void{
            m_log_tarea.text += "i socket_io_error" + a_event + "\n";
        }
        private function security_error(a_event:SecurityErrorEvent):void{
            m_log_tarea.text += "i socket_security_error" + a_event + "\n";
        }
        //--------------------------------------------------------------------//
    }
}