sudo env ARCHFLAGS="-arch i386" gem install mysql -- --with-mysql-include=/opt/local/include/mysql5 --with-mysql-lib=/opt/local/lib/mysql5 --with-mysql-config=/opt/local/lib/mysql5/bin/mysql_config
This worked for me with mysql installed via macports by the command:
sudo port install mysql5 +server
From Code Intensity.