A monkey patch for serving compressed assets for Rails 3.0 (asset_tag_helper.rb) and another for Rails 3.1 (asset_paths.rb).

assert_url_helper.rb

# Rails 4

require 'action_view/helpers/asset_url_helper'

module ActionView
  module Helpers
    module AssetUrlHelper
      def accept_encoding?(encoding)
        request = self.request if respond_to?(:request)
        return false unless request
        (request.env['HTTP_ACCEPT_ENCODING'] || '').split(',').include?(encoding)
      end

      def rewrite_path_to_gzip?(source, options)
        request = self.request if respond_to?(:request)
        return false unless request
        [ :javascript, :stylesheet ].include?(options[:type]) &&
          Rails.application.asset_url(request.try(:scheme) || 'http').present? &&
          accept_encoding?('gzip')
      end

      alias_method :_asset_path, :asset_path

      def asset_path(source, options = {})
        _asset_path(source, options).tap do |path|
          path << '.cgz' if rewrite_path_to_gzip?(source, options)
        end
      end

      alias_method :path_to_asset, :asset_path
    end
  end
end

asset_paths.rb

# Rails 3

module ActionView
  class AssetPaths
    
    def accept_encoding?(encoding)
      return false unless current_request
      (current_request.env['HTTP_ACCEPT_ENCODING'] || '').split(',').include?(encoding)
    end
  
    def rewrite_path_to_gzip?(source, options)
      [ "js", "css" ].include?(options[:ext]) and
        (! config.asset_host.blank?) and
        (source =~ /assets\//) and
        accept_encoding?('gzip')
    end

    def rewrite_path_to_gzip(source)
      source + ".cgz"
    end
    
    alias_method :actionview_assetpaths_compute_public_path, :compute_public_path
    def compute_public_path(source, dir, options = {})
      source = rewrite_path_to_gzip(source) if rewrite_path_to_gzip?(source, options)
      actionview_assetpaths_compute_public_path(source, dir, options)
    end

  end
end

asset_tag_helper.rb

# Rails 3

module ActionView
  module Helpers
    module AssetTagHelper
      def accept_encoding?(encoding)
        (request.env['HTTP_ACCEPT_ENCODING'] || '').split(',').include?(encoding)
      end
      def rewrite_path_to_gzip?(source)
        (! config.asset_host.blank?) and (source =~ /assets\//) and accept_encoding?('gzip')
      end
      def path_to_javascript(source)
        source = rewrite_path_to_gzip(source) if rewrite_path_to_gzip?(source)
        compute_public_path(source, 'javascripts', 'js')
      end
      def path_to_stylesheet(source)
        source = rewrite_path_to_gzip(source) if rewrite_path_to_gzip?(source)
        compute_public_path(source, 'stylesheets', 'css')
      end
      def rewrite_path_to_gzip(source)
        source + ".cgz"
      end
    end
  end
end

asset_tag_helper_spec.rb

# Rails 3

require 'spec_helper'

describe ActionView::Helpers::AssetTagHelper do
  before(:each) do
    @helper = SpecAssetTagHelper.new
  end
  context "accept_encoding?" do
    it "works for an empty string" do
      @helper.accept_encoding?('gzip').should be_false
    end
    it "works for one encoding that is not accepted" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'foo'
      @helper.accept_encoding?('gzip').should be_false
    end
    it "works for two encodings that are not accepted" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'foo,bar'
      @helper.accept_encoding?('gzip').should be_false
    end
    it "works for one encoding that is accepted" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'gzip'
      @helper.accept_encoding?('gzip').should be_true
    end
    it "works for two encodings when one is accepted" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'foo,gzip'
      @helper.accept_encoding?('gzip').should be_true
    end
  end
  context "rewrite_path_to_gzip?" do
    it "works for no headers and a non-asset path" do
      @helper.rewrite_path_to_gzip?('foo').should be_false
    end
    it "doesn't rewrite non-asset paths" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'gzip'
      @helper.rewrite_path_to_gzip?('server/bar/foo.css').should be_false
    end
    it "rewrites asset paths" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'gzip'
      @helper.rewrite_path_to_gzip?('server/assets/foo.css').should be_true
    end
  end
  context "path_to_javascript" do
    it "rewrites asset path" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'gzip'
      @helper.path_to_javascript('/assets/javascripts/example.js').should == "http://stagic.example.com/assets/javascripts/example.js.cgz"
    end
  end
  context "path_to_stylesheet" do
    it "rewrites asset path" do
      @helper.env['HTTP_ACCEPT_ENCODING'] = 'gzip'
      @helper.path_to_stylesheet('/assets/stylesheets/example.css').should == "http://stagic.example.com/assets/stylesheets/example.css.cgz"
    end
  end
end