<?xml version="1.0" encoding="iso-8859-1"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">

  <mx:Panel id="painel" title="Estudos" status="Active" 
     height="300" width="200" paddingTop="10" 
     paddingLeft="10" paddingRight="10" 
     paddingBottom="10">

     <mx:TextInput change="habilitar(event)" id="texto"/>
     <mx:Button label="OK" enabled="false" id="btnOK"/>

  </mx:Panel>

  <mx:Script>
    <![CDATA[
      import mx.utils.StringUtil;      

      private function habilitar(evento:Event):void{
        if(StringUtil.trim(texto.text) == "")
          btnOK.enabled = false;
        else
          btnOK.enabled = true;        
      }
    ]]>
  </mx:Script>

</mx:Application>