[Vlasenko Fedor]

tsigel,
а без valueOf?

а разве это было в задании ?

function sum() {

  var currentSum = getSum(arguments);

  function fn() {
    currentSum += getSum(arguments);
    return fn;
  }
  function getSum(args) {
 		var argSum = 0;
	    for (var i = 0; i <args.length; i++) {
	  		argSum += args[i]
	  	};
    return argSum;
  }
  fn.toString = fn.valueOf = function() {
    return currentSum;
  };
  return fn;
}

alert( 'sum(2, 5, 10) =  ' + sum(2, 5, 10) + "\n" 
+ 'sum(2, 5)(10) =  ' + sum(2, 5)(10) + "\n" 
+ 'sum(2)(5)(10) = ' + sum(2)(5)(10) + "\n" 
+ 'sum(2)(5, 10) = ' + sum(2)(5, 10));

Если количество вызываемых аргументов равно 3

function sum() {

    var currentSum = getSum(arguments);

    function fn() {
        currentSum += getSum(arguments);
        return checked(arguments.length);
    }

    function getSum(args) {
        var argSum = 0;
        for (var i = 0; i < args.length; i++) {
            argSum += args[i]
        }

        return argSum;
    }

    function checked(col) {
        fn.arg = fn.arg || 0;
        fn.arg += col;
        return fn.arg == 3 ? currentSum : fn;
    }

    return checked(arguments.length);

}

alert( 'sum(2, 5, 10) =  ' + sum(2, 5, 10) + "\n" 
+ 'sum(2, 5)(10) =  ' + sum(2, 5)(10) + "\n" 
+ 'sum(2)(5)(10) = ' + sum(2)(5)(10) + "\n" 
+ 'sum(2)(5, 10) = ' + sum(2)(5, 10));