There are a few binary Emacs packages for OS X floating around out there, but I always build it myself from the sources. This usually results in an Emacs that works the way I expect, rather than the way some "helpful" distributor thinks it ought to work.
I'll assume you have the developer tools and bzr installed, and know how to open Terminal and type some commands. Here are the commands you need to type:
bzr init-repo --2a emacs/
cd emacs
bzr branch bzr://bzr.savannah.gnu.org/emacs/trunk/
cd trunk
./configure --with-ns
make install
When this is complete, you'll end up with Emacs.app in the nextstep subdirectory. You can run Emacs.app from there, or copy it to your Applications directory.
Update 2010/10/29: Discovered that the Emacs team now uses Bazaar (bzr) rather than CVS. Updated the instructions accordingly, following advice from http://www.emacswiki.org/emacs/EmacsForMacOS and http://www.emacswiki.org/emacs/BzrForEmacsDevs. Also, found what appears to be a faithful binary distribution at http://emacsformacosx.com/.