Login Failure when Testing with Capybara, Rspec and Selenium in Rails 3.1

Question

I added some confirmation dialog boxes for my Rails 3.1 application and, prior to that, their corresponding tests. Following the model of Railscast #257, I added ':js => true' to the test, added database_cleaner and modified the spec_helper.rb file accordingly.

When I run the test, Firefox launches, Capybara-Selenium fills in the fields the appropriate username and a password, but log-in fails (i.e., "invalid username/password".) Other tests that do not have ':js => true' and also log in, do still pass.

I would like to add more javascript to my application in the future and I am avoiding solutions that would hack Capybara to get this to work (e.g., click 'OK' on all dialogs.)

Any ideas what I might be missing? Fail that, any suggestions on how to debug this problem?

Thank you.