The current version of shoulda (2.10.3) is not compatible with Rails 3 and Bundler. It’s not that hard to make it work though. In your Gemfile:
group :test do
gem ‘shoulda’, :git => ‘git://github.com/thoughtbot/shoulda.git’, :branch => ‘rails3’, :require => nil
The “:require => nil” means shoulda will not be automatically loaded. To load it, simply require it after the other requires on test_helper.rb:
Voilá! You should have shoulda working with rails3 then.