Interesting that this came up today.  I've just started writing an xmonad layout manager that asks emacs where a certain buffer is physically on the screen, and then it puts a regular window on top of that.  (It will also let you emacs focus and unfocus that window, so pop-to-buffer will move xmonad's focus there, if necessary; and mod-j/mod-k will delegate to emacs when necessary.)<p>Anyway, the idea is to let you have a full-screen emacs with a real xterm or web browser "inside" of it.