init.rb

#don't use sinatra/main. we don't want the kitchen sink, just pretty rack apps
require 'sinatra/base'

class SinatraRailsPlugin < Sinatra::Base

  get '/foo' do
    'bar'
  end

  #this hackeration is required for sinatra to be a nice rack citizen
  error 404 do
    @app.call(env)
  end

end

config.after_initialize do
  config.gem 'sinatra'
  config.middleware.use SinatraRailsPlugin
end