From Google groups discussion:

Slightly off topic, but is there a reason you followed that convention 
for button menus?  I'm looking at making a patch to support custom 
menu types and I see two ways of doing it.  1) Add a type attribute to 
the menu element.  2) Replace the menu element with simply accepting 
<menu:Menu> elements so <menu:ColorMenu> and the like could be used. 
1 sticks with your convention for border layout, which would be good 
if there was a specific reason for it.  2 follows the general usage of 
UiBinder which you also use for MenuBar.