The best place to keep application constants which are environment specific is in config/environments directory. For instance...

# in RAILS_ROOT/config/environments/development.rb
APP_DOMAIN = "localhost"
# in RAILS_ROOT/config/environments/production.rb
APP_DOMAIN = "real-domain.com"

...will set the APP_DOMAIN constant to either "localhost" or "real-domain.com" depending on which environment Rails boots up.

Just finishing up brewing up some fresh ground comments...