Instrumenting rspec and using a custom formatter to generate API docs from specs. See http://blog.carbonfive.com/2011/04/21/generating-documentation-from-specs/

api_docs.rake

require 'rubygems'
require 'rspec/core/rake_task'

namespace :project_name do
  namespace :api do
    desc 'Generate API request documentation from API specs'
    RSpec::Core::RakeTask.new('generate_docs') do |t|
      t.pattern = 'spec/controllers/api/**/*_spec.rb'
      t.rspec_opts = ["-Ilib/tasks/api_docs", "-rinstrument_specs", "-rapi_formatter", "--format APIFormatter"]
    end
  end
end

api_formatter.rb

require 'rspec/core/formatters/base_formatter'

class APIFormatter < RSpec::Core::Formatters::BaseFormatter
  def initialize(output)
    super(output)
  end

  def example_group_started(example_group)
    indented_message("#{example_group.description}\n", example_group)
  end

  def example_group_finished(example_group)

  end

  def example_started(example)
    indented_message("#{example.description}\n", example)
  end

  def example_passed(example)
    indented_message("#{example.metadata[:curl]}\n")
    indented_message("#{example.metadata[:response]}\n\n")
  end

  def example_failed(example)

  end

  def example_pending(example)

  end

  private

  def indented_message(message, context = nil)
    if context.present?
      if context.respond_to?(:example_group)
        (context.example_group.ancestors.count + 1).times do
          output << '#'
        end
      else
        context.ancestors.count.times do
          output << '#'
        end
      end
    else
      output << '    '
    end
    output << message
  end
end

instrument_specs.rb

require 'action_dispatch'
require 'action_controller'

module InstrumentSpecs
  module ::ActionController::TestCase::Behavior
    alias_method :process_without_logging, :process

    def process(action, parameters = nil, session = nil, flash = nil, http_method = 'GET')
      version, route = described_class.to_s.match(/::(V\d)::(\S+)Controller/)[1..2].collect(&:downcase)
      curl = "curl "
      parameters.each {|key, value| curl << "-d \"#{key}=#{CGI.escape(value)}\" " } if parameters.present?
      curl << "-X #{http_method} "
      curl << "https://api.host.example/#{version}/#{route} "
      example.metadata[:curl] = curl

      response = process_without_logging(action, parameters, session, flash, http_method)
      example.metadata[:response] = "#{response.status} #{response.body}"

      response
    end
  end
end