<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/03/19/horizontally-aligning-tabs-within-a-tabbar-control-in-flex/ -->
<mx:Application name="TabBar_horizontalAlign_test"
        xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.events.ListEvent;

            private function comboBox_change(evt:ListEvent):void {
                tabBar.setStyle("horizontalAlign", comboBox.selectedItem.label);
            }
        ]]>
    </mx:Script>

    <mx:Array id="arr">
        <mx:Object label="Button" />
        <mx:Object label="ButtonBar" />
        <mx:Object label="ColorPicker" />
        <mx:Object label="ComboBox" />
    </mx:Array>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="horizontalAlign:">
                <mx:ComboBox id="comboBox"
                        change="comboBox_change(event);">
                    <mx:dataProvider>
                        <mx:Array>
                            <mx:Object label="left" />
                            <mx:Object label="center" />
                            <mx:Object label="right" />
                        </mx:Array>
                    </mx:dataProvider>
                </mx:ComboBox>
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:Box backgroundColor="haloSilver">
        <mx:TabBar id="tabBar"
                dataProvider="{arr}"
                width="600"
                tabWidth="100"
                tabHeight="40"
                direction="horizontal" />
    </mx:Box>

</mx:Application>