Add the following line to your config/environment.rb file:
ActionController::Base.asset_host = 'http://assets.yourdomain.com'
It's also possible to set a single environment to use an external asset host:
Add the following line to config/environments/production.rb (the most likely spot - this line should already be in this file, but commented out):
config.action_controller.asset_host = 'http://assets.yourdomain.com'
Now this line in my layout
<%= stylesheet_link_tag 'scaffold/base' %>
will translate to http://assets.yourdomain.com/stylesheets/scaffold/base.css
Additional information on setting up multiple asset hosts can be found at: