Have a look at addArrayObserver, see http://jsfiddle.net/pangratz666/EE65Z/:
var a = Ember.A('a b c d e f g'.w());
var o = Ember.Object.create({
arrayWillChange: Ember.K,
arrayDidChange: function(array, start, removeCount, addCount) {
console.log(arguments);
}
});
a.addArrayObserver(o);