gistfile1.txt

<label for='field' tooltip='hello world'>Bla bla</label>

Becomes:

<span class='tooltip'>
 <label for='field' tooltip='hello world'>Bla bla</label>
 <div class='something'>hello world</div>
</span>

something like this :
Haven't tested it...

directive('tooltip', function () {
  return {
    restrict: 'A',
    replace: true,
    transclude: true,
    scope: {
      tooltip: '@'
    },
    template: 
    "<span class='tooltip'>" +
    "<div ng-transclude></div>" +
    "<div class='something'>{{tooltip}}</div>" +
    "</span>"

  };
});