As of early 2009 the combination of Cucumber and Webrat is gaining acceptance as a best-practice in acceptance testing for Rails applications. Cucumber an english-like syntax and more importantly a structure for thinking about the behavior of the application. Webrat provides a mechanism for specifying interactions with the application at a high level, and can be connected to Selenium for testing ajax functionality that requires real browser interaction.
On a technical note, Cucumber uses "step definitions" written in ruby to process the feature specs written in cucumber syntax. Given the thematic conistency of working with cards in wagn, I think that a wide range feature behavior can be specified with a fairly small number of step definitions, ie. the setup will be compact.
My estimate for developing a reasonably complete acceptance test suite is 3 developer days for setup (step definitions, initial tests, ajax/selenium setup), and 4-6 additional developer days for completing the spec. The latter part would be best done as pair coding in a sprint.
I would love to see this happen sometime in 2009 :)