Rails 3.beta don't work with rack-mount 0.5.2

If I force rack-mount 0.4 installation it works as expected.

@@@ Diego-Plentzs-Computer:teste plentz$ gem list

LOCAL GEMS

abstract (1.0.0)
actionmailer (3.0.0.beta)
actionpack (3.0.0.beta)
activemodel (3.0.0.beta)
activerecord (3.0.0.beta)
activeresource (3.0.0.beta)
activesupport (3.0.0.beta)
arel (0.2.1)
builder (2.1.2)
bundler (0.9.7)
erubis (2.6.5)
i18n (0.3.3)
mail (2.1.2)
memcache-client (1.7.8)
mime-types (1.16)
rack (1.1.0)
rack-mount (0.5.2)
rack-test (0.5.3)
rails (3.0.0.beta)
railties (3.0.0.beta)
rake (0.8.7)
rspec (2.0.0.a5)
rspec-core (2.0.0.a5)
rspec-expectations (2.0.0.a5)
rspec-mocks (2.0.0.a5)
rubygems-update (1.3.5)
text-format (1.0.0)
text-hyphen (1.0.0)
thor (0.13.3)
tzinfo (0.3.16)
Diego-Plentzs-Computer:teste plentz$ ruby -v
ruby 1.9.1p376 (2009-12-07 revision 26041) [i386-darwin10]
Diego-Plentzs-Computer:Projects plentz$ rails teste

  create
  create  README
  create  .gitignore
  create  Rakefile
  create  config.ru
  create  Gemfile
  create  app
  create  app/controllers/application_controller.rb
  create  app/helpers/application_helper.rb
  create  app/models
  create  app/views/layouts
  create  config
  create  config/routes.rb
  create  config/application.rb
  create  config/environment.rb
  create  config/environments
  create  config/environments/development.rb
  create  config/environments/production.rb
  create  config/environments/test.rb
  create  config/initializers
  create  config/initializers/backtrace_silencers.rb
  create  config/initializers/cookie_verification_secret.rb
  create  config/initializers/inflections.rb
  create  config/initializers/mime_types.rb
  create  config/initializers/session_store.rb
  create  config/locales
  create  config/locales/en.yml
  create  config/boot.rb
  create  config/database.yml
  create  db
  create  db/seeds.rb
  create  doc
  create  doc/README_FOR_APP
  create  lib
  create  lib/tasks
  create  lib/tasks/.gitkeep
  create  log
  create  log/server.log
  create  log/production.log
  create  log/development.log
  create  log/test.log
  create  public
  create  public/404.html
  create  public/422.html
  create  public/500.html
  create  public/favicon.ico
  create  public/index.html
  create  public/robots.txt
  create  public/images
  create  public/images/rails.png
  create  public/stylesheets
  create  public/stylesheets/.gitkeep
  create  public/javascripts
  create  public/javascripts/application.js
  create  public/javascripts/controls.js
  create  public/javascripts/dragdrop.js
  create  public/javascripts/effects.js
  create  public/javascripts/prototype.js
  create  public/javascripts/rails.js
  create  script
  create  script/rails
  create  test
  create  test/performance/browsing_test.rb
  create  test/test_helper.rb
  create  test/fixtures
  create  test/functional
  create  test/integration
  create  test/unit
  create  tmp
  create  tmp/sessions
  create  tmp/sockets
  create  tmp/cache
  create  tmp/pids
  create  vendor/plugins
  create  vendor/plugins/.gitkeep

Diego-Plentzs-Computer:Projects plentz$ cd teste/
Diego-Plentzs-Computer:teste plentz$ ls
Gemfile Rakefile config db lib public test vendor
README app config.ru doc log script tmp
Diego-Plentzs-Computer:teste plentz$ rails --help
/opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler/resolver.rb:52:in block in resolve': No compatible versions could be found for required dependencies: (Bundler::VersionConflict)

rack-mount (~> 0.4.0, runtime) not found in any of the sources
  required by rails (= 3.0.0.beta, runtime)
All possible versions of origin requirements conflict.
from /opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler/resolver.rb:39:in `catch'
from /opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler/resolver.rb:39:in `resolve'
from /opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler/runtime.rb:95:in `specs'
from /opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler/runtime.rb:81:in `specs_for'
from /opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler/runtime.rb:18:in `setup'
from /opt/local/lib/ruby/gems/1.9.1/gems/bundler-0.9.7/lib/bundler.rb:66:in `setup'
from /Users/plentz/Projects/criterium/teste/config/boot.rb:7:in `rescue in <top (required)>'
from /Users/plentz/Projects/criterium/teste/config/boot.rb:2:in `<top (required)>'
from /Users/plentz/Projects/criterium/teste/script/rails:9:in `require'
from /Users/plentz/Projects/criterium/teste/script/rails:9:in `<main>'