I have used this syntax in multiple places to dynamically generate selection-box contents from AJAX feedback.

Code:
myList.options[myList.options.length] = new Option("Test",123);
It works in every case except when I define "myList" using an ID from the "opener"... as in:

Code:
var myList = opener.document.getElementById("testID");
I have even gone as far as to try the old MS IE compatibility way:

Code:
var myList = opener.document.all["testID"];