This Rails 3 app demonstrates how you can use Rack::Throttle to throttle any Rack app you have 
mounted under Rails. It also contains Cucumber tests to verify access and throttling to the API.

Modifications mainly made to these files:
* config/routes.rb
* features/my_dumb_api.feature
* features/step_definitions/api_steps.rb
* Gemfile

I wrote up a blog post detailing how to do all this step-by-step, see:
http://jamesbadger.ca/2012/01/14/rails-3-rack-throttle-your-rack-api/