If you're running on Passenger, then the default is to run in production, in your apache conf:
<VirtualHost *:80>
ServerName application_name.rails.local
DocumentRoot "/Users/rails/application_name/public"
RailsEnv production ## This is the default
</VirtualHost>
If you're just running a local server with mongrel or webrick, you can do:
./script/server -e production
or in bash:
RAILS_ENV=production ./script/server
actually overriding the RAILS_ENV constant in the enviornment.rb should probably be your last resort, as it's probably not going to stay set (see another answer I gave on that)