The newest version (24.3) of Emacs.app includes support for Mountain Lion's fullscreen. However, even after disabling the toolbar (tool-bar-mode 0), the space for the toolbar is not used by Emacs, it just remains blank until the menu is shown. How do I get this space back for a more full fullscreen experience?
The correct answer is a newer version of Emacs.app is required. 24.3, while the latest, is not the latest and greatest. 24.3.50 is available via a nightly build. It solves the problem.