Gemfile

source 'http://rubygems.org'
gem 'rails', '3.0.3'

group :development do
  gem 'ruby-prof', '< 0.9.0'
  gem 'rack-contrib' #, :require => 'rack/contrib'
end

middleware.rb

class MyTest::Application
  config.middleware.use ::Rack::Profiler, :printer => ::RubyProf::GraphHtmlPrinter
end

shell transcript

# create empty gemset
$ rvm use @bundler_108 --create

# install bundler and rails
$ gem install bundler -v 1.0.8
$ gem install rails

# create new rails project
$ rails new my_test

# change into my_test and edit Gemfile to match above version
$ cd my_test
$ vi Gemfile
$ vi config/initializers/middleware.rb

# Install gems and test app
$ bundle install
$ rails s
=> Booting WEBrick
=> Rails 3.0.3 application starting in development on http://0.0.0.0:3000
=> Call with -d to detach
=> Ctrl-C to shutdown server
Exiting
/private/tmp/my_test/config/initializers/middleware.rb:2: uninitialized constant Rack::Profiler (NameError)
/private/tmp/my_test/config/initializers/middleware.rb:2: uninitialized constant Rack::Profiler (NameError)
....