Check out this logic
<?xml version="1.0" encoding="utf-8"?>
<fx:Script>
<![CDATA[
protected function button1_mouseOverHandler(event:MouseEvent):void
{
var target:Button = event.target as Button;
if ( target.filters == null )
target.toolTip = null;
else
target.toolTip = "Here is the disabled Icon";
}
]]>
</fx:Script>
<fx:Declarations>
<!-- Place non-visual elements (e.g., services, value objects) here -->
</fx:Declarations>
<mx:Button label="Disabled Icon" mouseOver="button1_mouseOverHandler(event)" />
</s:Application>