function fixFooter() {
  var body = $('window');
  body.setStyle({height: ''});
  
  if (body.getHeight() < document.viewport.getHeight()) {
    body.setStyle('height: ' + document.viewport.getHeight() + 'px');
  }
}
Event.observe(window, 'resize', fixFooter);
Event.observe(window, 'load', fixFooter);