Autotest would constantly run my tests even though I hadn’t modified anything. Which made it impossible to actually use. Creating the following in a file called .autotest in my application’s root solved the problem for me. This is on Rails 3 RC, RSpec 2 beta 19 and Cucumber 0.8.5.

  Autotest.add_hook :initialize do |at|
    at.add_exception(%r{^\./\.git})
    at.add_exception(%r{^\./db})
    at.add_exception(%r{^\./log})
    at.add_exception(%r{^\./tmp})
    at.add_exception(%r{^\./rerun\.txt})
    at.add_exception(%r{^\./Gemfile\.lock})
  end

UPDATE: Updated with Wes' suggestion in the comments.