EDIT: Looks like emacsforosx does not compile Emacs with support for above command. Will you consider it? I totally understand distributing plain vanilla version though!

But it's frustrating for me, too, because I want the feature as much as anyone else. I've been making do with "(setq ns-auto-hide-menu-bar t)" which is close to the desired behavior, especially when combined with maxframe [1]. It still has the window's title bar, so it's not exactly there.

[1] https://github.com/rmm5t/maxframe.el I use it like this (where super is the command key):

  (defun toggle-frame-maximization ()
    (interactive)
    (if (frame-parameter nil 'mf-maximized) (restore-frame) (maximize-frame)))
  (define-key global-map [(super return)] 'toggle-frame-maximization)

  brew update  
  # Lion only (or if you don't have xcode?)  
  brew tap homebrew/dupes  
  brew install apple-gcc42  
  # put these in your .bashrc  
  export HOMEBREW_USE_GCC=1  
  export HOMEBREW_VERBOSE=1  
  # /Lion only  
  brew install emacs --cocoa  
  brew linkapps