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I've added always handler. Now the code looks like this (thanx to Flambino)

function defer() {
 var currentState = 'pending',
 callbacks = { resolved: [], rejected: [], always: [] },
 args = []; //
 function execute(state) {
 var cb;
 while(callbacks[state].length) {
 cb = callbacks[state].shift();
 if( typeof cb === 'function' ) cb.apply(this, args);
 }
 }
 // generic function factory for done/fail functions
 function handle(state) {
 return function (cb) {
 callbacks[state].push(cb);
 if(currentState !== 'pending') {
 if(currentState === state) execute(state);
 if(state === 'always') execute('always');
 }
 }
 }
 // generic function factory for resolve/reject functions
 function complete(state) {
 return function () {
 if(currentState !== 'pending') return;
 args = Array.prototype.slice.call(arguments, 0);
 currentState = state;
 execute(state);
 execute('always');
 }
 }
 return {
 promise: function () {
 return {
 done: handle('resolved'),
 fail: handle('rejected'),
 always: handle('always')
 };
 },
 resolve: complete('resolved'),
 reject: complete('rejected')
 };
}
 
 

Very useful thing (missed in initial version):
args = Array.prototype.slice.call(arguments, 0);
This line of code magically converts arguments from object into array