install_mcrypt.sh

#!/usr/bin/sh

# Scipt for installing mcrypt.so extension for PHP 5.3.13 (default PHP for OS X 10.8 Mountain Lion)

mkdir /tmp/mcrypt
cd /tmp/mcrypt

curl --location --progress-bar http://sourceforge.net/projects/mcrypt/files/Libmcrypt/2.5.8/libmcrypt-2.5.8.tar.gz/download | tar -zx
curl --location --progress-bar http://us.php.net/get/php-5.3.26.tar.gz/from/us.php.net/mirror | tar -zx

cd /tmp/mcrypt/libmcrypt-2.5.8 
MACOSX_DEPLOYMENT_TARGET=10.8 CFLAGS='-O3 -fno-common -arch i386 -arch x86_64' LDFLAGS='-O3 -arch i386 -arch x86_64' CXXFLAGS='-O3 -fno-common -arch i386 -arch x86_64'  ./configure --disable-dependency-tracking
make -j6   
sudo make install

cd /tmp/mcrypt/php-5.3.26/ext/mcrypt/
phpize   
MACOSX_DEPLOYMENT_TARGET=10.8 CFLAGS='-O3 -fno-common -arch i386 -arch x86_64' LDFLAGS='-O3 -arch i386 -arch x86_64' CXXFLAGS='-O3 -fno-common -arch i386 -arch x86_64'  ./configure --with-php-config=/usr/bin/php-config 
make -j6
sudo make install 

cd ~ && rm -rf /tmp/mcrypt

echo "Don't forget to add 'extension=mcrypt.so' to /etc/php.ini"

added a little more here: https://gist.github.com/phillpafford/8420567