In a comment on a previous post I wrote about Enhancing CFWindow with JavaScript, Nathan asked the question:
The answer is to use the Ext.BasicDialog.center() function, then get the windows Ext.Element object using the Ext.BasicDialog.getEl() function. The Ext.Element object has the getX() and getY() methods, which will give you access to the windows X and Y coordinates. You can use those, in combination with the Ext.BasicDialog.moveTo() function, to move the window to your desired position relative to the center.