app.rb

require 'sass'

configure do
  Tilt.register 'scss', Tilt::SassTemplate
  set :scss, :syntax => :scss
end

get '/stylesheets/:file.css' do |file|
  content_type 'text/css', :charset => 'utf-8'

  # no #scss method is defined (and #sass only looks for .sass files)
  # we must call render ourself:
  render :scss, file.to_sym, :layout => false, :views => './public/stylesheets'
end

config.ru

require 'sass/plugin/rack'

Sass::Plugin.options[:template_location] = 'public/stylesheets'
use Sass::Plugin::Rack