I just thought I'd add to this thread since there doesn't seem to be a solution available on the forums yet.
EDIT: THIS IS FOR EXTJS. I DID NOT REALIZE THIS WAS THE GXT FORUM WHEN I POSTED IT.
I just used the each() method of Ext.WindowMgr. In my case, I am using a button to retrieve info that I want about each window so I can store size and position in a database. Here's my button object:
You could adapt this code to be a general function to return an array of all open windows rather than storing the attributes.
,text: 'Save Layout'
var height = win.getHeight();
var width = win.getWidth();
var xpos = win.getPosition(true);
var ypos = win.getPosition(true);
//Do AJAXy stuff here