Problem
When trying to use cucumber in a new rails 3.0.7 application you get the following error when running rake cucumber for the first time:
uninitialized constant Capybara (NameError)

Solution
As cucumber is moving to using capybara instead of webrat as default, if you follow the instructions in the Rspec book for setting up you would get the above error.
What you would need to do is to replace webrat with capybara.
You can do that by commenting out /removing webrat from your Gemfile and then adding the following:

gem 'capybara'
gem 'database_cleaner'