config.ru

#\ -p 4000

gem 'activesupport', '2.3.5'
gem 'serve', '0.11.2'

require 'serve'
require 'serve/rack'

root = File.dirname(__FILE__)

use Rack::CommonLogger
use Rack::ShowStatus
use Rack::ShowExceptions

if ENV['SERVER_SOFTWARE'] =~ /passenger/i
  run Serve::RackAdapter.new(root + '/views')
else
  run Rack::Cascade.new([
    Serve::RackAdapter.new(root + '/views'),
    Rack::Directory.new(root + '/public')
  ])
end

Of course, just like any other Rack app, if you are using serve with Passenger you will still need a public/ directory, a tmp/ directory, and a tmp/restart.txt.

See:
http://www.modrails.com/documentation/Users%20guide.html#_deploying_a_rack_based_ruby_application