Jun 13 
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.