<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/06/19/setting-the-label-placement-on-a-radiobuttongroup-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

<mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="labelPlacement:">
                <mx:ComboBox id="comboBox"
                        dataProvider="[left, right, top, bottom]"
                        selectedIndex="1" />
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

<mx:RadioButtonGroup id="radioGroup"
            labelPlacement="{comboBox.selectedItem}" />

<mx:HBox horizontalAlign="left" verticalGap="15">
        <mx:RadioButton id="radioButton1"
                label="Red"
                group="{radioGroup}"
                opaqueBackground="0xFF00FF" />
        <mx:RadioButton id="radioButton2"
                label="Orange"
                group="{radioGroup}"
                opaqueBackground="0xFF00FF" />
        <mx:RadioButton id="radioButton3"
                label="Yellow"
                group="{radioGroup}"
                opaqueBackground="0xFF00FF" />
        <mx:RadioButton id="radioButton4"
                label="Green"
                group="{radioGroup}"
                opaqueBackground="0xFF00FF" />
        <mx:RadioButton id="radioButton5"
                label="Blue"
                group="{radioGroup}"
                opaqueBackground="0xFF00FF" />
    </mx:HBox>

</mx:Application>