From http://jaredonline.github.com/blog/2012/05/16/sprockets-2-with-rails-2-dot-3/

asset_tag_helper.rb

# Overwrite some Asset Tag Helpers to use Sprockets
module ActionView
  module Helpers

    # Overwrite the javascript_path method to use the 'assets' directory
    # instead of the default 'javascripts' (Sprockets will figure it out)
    def javascript_path(source, options={})
      path = compute_public_path(source, 'assets', options.merge(:ext => 'js'))
      options[:body] ? path + "?body=1" : path
    end
    alias_method :path_to_javascript, :javascript_path # aliased to avoid conflicts with a javascript_path named route


    # Overwrite the stylesheet_path method to use the 'assets' directory
    # instead of the default 'stylesheets' (Sprockets will figure it out)
    def stylesheet_path(source, options={})
      path = compute_public_path(source, 'assets', options.merge(:ext => 'css'))
      options[:body] ? path + "?body=1" : path
    end
    alias_method :path_to_stylesheet, :stylesheet_path # aliased to avoid conflicts with a stylesheet_path named route
  end

  # Overwrite the stylesheet_link_tag method to expand sprockets files if 
  # debug mode is turned on.  Never cache files (like the default Rails 2.3 does).
  # 
  def stylesheet_link_tag(*sources)
    options = sources.extract_options!.stringify_keys
    debug   = options.key?(:debug) ? options.delete(:debug) : debug_assets?

    sources.map do |source|
      if debug && !(digest_available?(source, 'css')) && (asset = asset_for(source, 'css'))
        asset.to_a.map { |dep| stylesheet_tag(dep.logical_path, { :body => true }.merge(options)) }
      else
        sources.map { |source| stylesheet_tag(source, options) }
      end
    end.uniq.join("\n").html_safe
  end

  # Overwrite the javascript_include_tag method to expand sprockets files if 
  # debug mode is turned on.  Never cache files (like the default Rails 2.3 does).
  #
  def javascript_include_tag(*sources)
    options = sources.extract_options!.stringify_keys
    debug   = options.key?(:debug) ? options.delete(:debug) : debug_assets?

    sources.map do |source|
      if debug && !(digest_available?(source, 'js')) && (asset = asset_for(source, 'js'))
        asset.to_a.map { |dep| javascript_src_tag(dep.logical_path, { :body => true }.merge(options)) }
      else
        sources.map { |source| javascript_src_tag(source.to_s, options) }
      end
    end.uniq.join("\n").html_safe
  end

  private

  def javascript_src_tag(source, options)
    body = options.has_key?(:body) ? options.delete(:body) : false
    content_tag("script", "", { "type" => Mime::JS, "src" => path_to_javascript(source, :body => body) }.merge(options))
  end

  def stylesheet_tag(source, options)
    body = options.has_key?(:body) ? options.delete(:body) : false
    tag("link", { "rel" => "stylesheet", "type" => Mime::CSS, "media" => "screen", "href" => html_escape(path_to_stylesheet(source, :body => body)) }.merge(options), false, false)
  end

  def debug_assets?
    Rails.configuration.action_view.debug_sprockets || false
  end

  # Add the the extension +ext+ if not present. Return full URLs otherwise untouched.
  # Prefix with <tt>/dir/</tt> if lacking a leading +/+. Account for relative URL
  # roots. Rewrite the asset path for cache-busting asset ids. Include
  # asset host, if configured, with the correct request protocol.
  def compute_public_path(source, dir, options = {})
    source = source.to_s
    return source if is_uri?(source)

    source = rewrite_extension(source, options[:ext]) if options[:ext]
    source = rewrite_asset_path(source, dir, options)
    source = rewrite_relative_url_root(source, ActionController::Base.relative_url_root)
    source = rewrite_host_and_protocol(source, options[:protocol])
    source
  end

  def rewrite_relative_url_root(source, relative_root_url)
    relative_root_url && !(source =~ Regexp.new("^" + relative_root_url + "/")) ? relative_root_url + source : source
  end

  def has_request?
    @controller.respond_to?(:request)
  end

  def rewrite_host_and_protocol(source, porotocol = nil)
    host = compute_asset_host(source)
    if has_request? && !host.blank? && !is_uri?(host)
      host = @controller.request.protocol + host
    end
    host ? host + source : source
  end

  # Check for a sprockets version of the asset, otherwise use the default rails behaviour.
  def rewrite_asset_path(source, dir, options = {})
    if source[0] == ?/
      source
    else
      source = digest_for(source.to_s)
      source = Pathname.new("/").join(dir, source).to_s
      source
    end
  end

  def digest_available?(logical_path, ext)
    (manifest = Sprockets.manifest) && (manifest.assets[logical_path + "." + ext])
  end

  def digest_for(logical_path)
    if (manifest = Sprockets.manifest) && (digest = manifest.assets[logical_path])
      digest
    else
      logical_path
    end
  end

  def rewrite_extension(source, ext)
    if ext && File.extname(source) != "." + ext
      source + "." + ext
    else
      source
    end
  end

  def is_uri?(path)
    path =~ %r{^[-a-z]+://|^(?:cid|data):|^//}
  end

  def asset_for(source, ext)
    source = source.to_s
    return nil if is_uri?(source)
    source = rewrite_extension(source, ext)
    Sprockets.env[source]
  rescue Sprockets::FileOutsidePaths
    nil
  end
end