tiny Promise.js

function Promise () {
	this._thens = [];
}

Promise.prototype = {

	then: function (resolve, reject) {
		// capture calls to then()
		this._thens.push({ resolve: resolve, reject: reject });
	},

	resolve: function (val) { this._complete('resolve', val); },

	reject: function (ex) { this._complete('reject', ex); },

	_complete: function (which, arg) {
		// switch over to sync then()
		this.then = which === 'resolve' ?
			function (resolve, reject) { resolve(arg); } :
			function (resolve, reject) { reject(arg); };
		// complete all async then()s
		var aThen, i = 0;
		while (aThen = this._thens[i++]) { aThen[which] && aThen[which](arg); }
		delete this._thens;
	}
	
};

Just after you forked this, I added a line that will throw an exception if the resolve() or reject() methods are called more than once. That should save some tedious debug time. :)

Thanks for letting me know! Appreciate the update :)