<script>
  function GetText()
  {
    if (!document.all && !document.all.item) 
	  return "";
    var oSelTxt = document.selection.createRange();
	return oSelTxt.text;
  }
</script>

<form name="forma">
<textarea name="message">Some message</textarea><br>
<input type="button" onclick="alert(GetText())" value="GO!">
</form>
Some text