function observeSelect(reg){
  $$(reg+' select.get').each(function(element){
    // prüft, ob es evtl. schon observed wurde.... (muss aber nicht)
    if(!element.observingChange){
      element.observe('change', function(event){
         el = event.element();
	 // do something
      });
      element.observingChange = true;
    };
  });
}

document.observe("dom:loaded", function(){
  observeSelect('#filter')  // observed alle filter selects
});