AMD Observer module, depends on EventEmitter (see my other gists). (235 bytes minified, gzipped)

Observer.js

define(
	'Observer',
	[
		'EventEmitter'
	],
	function(EventEmitter) {
		'use strict';

		function copyValue(value) {
			if (typeof value === 'object') {
				return value instanceof Array
					? value.slice() 
					: Object.create(value);
			}

			return value;
		}
		
		return function(model) {
			model = model || {};
 
			return EventEmitter({
				get: function(key) {
					return copyValue(model[key]);
				},
				set: function(key, value) {
					var change = {
						type: 'updated',
						object: this,
						name: key,
						oldValue: copyValue(model[key]),
						value: copyValue(value)
					};

					model[key] = value;
					this.trigger('change', change);
				}
			});
		};
	}
);

Observer.min.js

define("Observer",["EventEmitter"],function(c){function d(a){return"object"===typeof a?a instanceof Array?a.slice():Object.create(a):a}return function(a){a=a||{};return c({get:function(b){return d(a[b])},set:function(b,e){var c={type:"updated",object:this,name:b,oldValue:d(a[b]),value:d(e)};a[b]=e;this.trigger("change",c)}})}});