In Emacs HEAD it is not possible to disable ATSUI font rendering on
Mac OS X at run-time or at compile-time without hacking the source.

   $ grep -re 'define[[:space:]]\+USE_ATSUI' emacs
   emacs/src/macgui.h:#define USE_ATSUI 1
   $

IMHO, and personal preference nonwithstanding, rendering without ATSUI
should at least be an option at compile-time.  I've attached an image
to demonstrate that it delivers a crispness that is not matched by its
ATSUI counterparts (top: USE_ATSUI=1, mac-allow-anti-aliasing=t;
center: USE_ATSUI=1, mac-allow-anti-aliasing=nil; bottom: USE_ATSUI=0;
all using "-*-Monaco-*-100-*").

In the spirit of offering choices I would suggest introducing a
--disable-atsui configure option, and I'd be happy to send over a
corresponding patch I have sitting here.  It just defines USE_ATSUI to
be 0 if the option is specified.

Unfortunately I don't have knowledge about OS X font rendering APIs or
Emacs source internals, or I would be preparing a patch that allows
switching the rendering engine at run-time.

Julian Scheid