Sweet! I found a link on emacswiki to a blog post on building emacs with fullscreen ability. Unfortunately, the code base is months old. Fortunately Steve Purcell posted instructions for merging the fullscreen mods into the current codebase. Nirvana!
Check this out:
So it's a bit hard to tell, but that is fullscreen: no menu bar. Yay! And notice the sweet transparency. Yeah, it's in there.
I've added my copy of the patched git repository to my site as well as the binary I created. Find them here.
The command for toggling fullscreen mode on OS X is:
M-x ns-toggle-fullscreen. If you want to bind it to a key sequence, add something like the code below to your .emacs file (I use M-return).
(global-set-key (kbd "M-RET") 'ns-toggle-fullscreen)