To supplement this solution, I provide alternative solutions:
You can apply a colormatrixfilter on the main icon when the button is disabled. The main advantage is that you don’t need to embed another icon (only if you really require a different icon for disabled button) and the rendering is similar : everything is gray, the button, the label and the icon also.
Check this links :
http://afoucal.free.fr/index.php/2008/01/25/imagebutton-gray-disabled/
Or the following for a more customizable icon with other kind of filter:
http://afoucal.free.fr/index.php/2008/12/03/flex-enhancedbutton-free-the-icon/

Regards

Does this work for you?

<mx:Script>
    <![CDATA[
        private function hasData(btn:Button):Boolean {
            return (btn.data != null) && String(btn.data).length > 0;
        }
    ]]>
</mx:Script>

<mx:Form>
    <mx:FormItem label="button 1 (default - null):">
        <mx:Button id="btn1"
                label="Que?"
                visible="{hasData(btn1)}" />
    </mx:FormItem>
    <mx:FormItem label="button 2 (empty):">
        <mx:Button id="btn2"
                data=""
                label="Que?"
                visible="{hasData(btn2)}" />
    </mx:FormItem>
    <mx:FormItem label="button 3 (defined):">
        <mx:Button id="btn3"
                data="value"
                label="Que?"
                visible="{hasData(btn3)}" />
    </mx:FormItem>
</mx:Form>

Peter

Thanks in Advacnce for the help
Hope to hear soon

Visi