function getSearchField()
    {
      return $$(".search_field")[0];  
    }
function searchFieldEntered()
    {
      getSearchField().value = "";
    }
function searchFieldBlurred()
    {
      el = getSearchField();
      if (el.value == "")
      {
        el.value = el.defaultValue;
      }
    }
function loaded()    
    {
      var el = getSearchField();
      Event.observe(el,"focus",searchFieldEntered);  
      Event.observe(el, "blur", searchFieldBlurred);
    }
Event.observe(window,"load",loaded)