confirm_dialog_steps.rb

Then /^(.+) and I confirm dialog box$/ do |step|
  bypass_confirm_dialog

  Then step
end

js_helpers.rb

module JsHelpers
  def bypass_confirm_dialog
    # bypassing the confirm dialog
    # http://groups.google.com/group/ruby-capybara/browse_thread/thread/89760b6fcab7fd19
    # http://stackoverflow.com/questions/2458632/how-to-test-a-confirm-dialog-with-cucumber
    page.evaluate_script('window.confirm = function() { return true; }')
  end
end

some.feature

@javascript
Scenario: Lorem ipsum
...
  When I press "Delete" and I confirm dialog box